Publishing type：Research paper (scientific journal)   Publisher：Physical Society of Japan  

DOI： 10.7566/jpsj.92.054704

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Authorship：Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：Physical Society of Japan  

DOI： 10.7566/jpsj.92.044801

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society ({ACS})  

DOI： 10.1021/acsomega.2c08289

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.tsf.2023.139749

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Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry (RSC)  

Anomalous local structure and magnetic moment of an inhomogeneous FeSe based superconductor indicating importance of the structure and magnetic correlations in these materials.

DOI： 10.1039/d3cp00004d

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Physical Society of Japan  

DOI： 10.7566/jpsj.91.091007

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.materresbull.2022.111896

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Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/physrevb.106.085114

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.106.085114/fulltext

Publishing type：Research paper (scientific journal)   Publisher：MDPI AG  

Layered metal nitride halides MNX (M = Ti, Zr, Hf; X = Cl, Br, I) have two polymorphs, including α- and β-forms, which have the FeOCl and SmSI structures, respectively. These compounds are band insulators and become metals and show superconductivity after electron doping by intercalating alkali metals between the layers. The superconductivity of β-form had been extensively characterized from decades ago, but it is not easy to consistently interpret all experimental results using conventional phonon-mediated Bardeen–Cooper–Schriefer mechanisms. The titanium compound TiNCl crystallizes only in the α-form structure. TiNCl also exhibits superconductivity as high as ~16 K after electron doping by intercalating metals and/or organic basis. It is important to compare the superconductivity of different M–N networks. However, α-form compounds are vulnerable to moisture, unlike β-form ones. The intercalation compounds are even more sensitive to humid air. Thus, there are few experimental studies on the superconducting mechanism of α-form, although it has been discussed for exotic Cooper-pairing mechanisms. This short review gathers the recent progress in experimental studies of TiNCl.

DOI： 10.3390/condmat7020033

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.7566/JPSJ.91.054602

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.tsf.2022.139210

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Publishing type：Research paper (scientific journal)   Publisher：MDPI AG  

We have investigated the local structure of BiS2-based layered materials by Bi L3-edge extended X-ray absorption fine structure (EXAFS) measurements performed on single crystal samples with polarization of the X-ray beam parallel to the BiS2 plane. The results confirm highly instable nature of BiS2 layer, characterized by ferroelectric like distortions. The distortion amplitude, determined by the separation between the two in-plane (Bi-S1) bonds, is found to be highest in LaO0.77F0.23BiS2 with ΔR∼0.26 Å and lowest in NdO0.71F0.29BiS2 with ΔR∼0.13 Å. Among the systems with intrinsic doping, CeOBiS2 shows smaller distortion (ΔR∼0.15 Å) than PrOBiS2 (ΔR∼0.18 Å) while the highest distortion appears for EuFBiS2 revealing ΔR∼0.22 Å. It appears that the distortion amplitude is controlled by the nature of the RE(O,F) spacer layer in the RE(O,F)BiS2 structure. The X-ray absorption near edge structure (XANES) spectra, probing the local geometry, shows a spectral weight transfer that evolves systematically with the distortion amplitude in the BiS2-layer. The results provide a quantitative measurements of the local distortions in the instable BiS2-layer with direct implication on the physical properties of these materials.

DOI： 10.3390/physchem1030019

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.apmt.2021.101167

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Authorship：Last author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

Abstract

We have performed soft x-ray spectroscopy in order to study the photoirradiation time dependence of the valence band structure and chemical states of layered transition metal nitride chloride TiNCl. Under the soft x-ray irradiation, the intensities of the states near the Fermi level (E_F) and the Ti³⁺ component increased, while the Cl 2p intensity decreased. Ti 2p–3d resonance photoemission spectroscopy confirmed a distinctive Fermi edge with Ti 3d character. These results indicate the photo-induced metallization originates from deintercalation due to Cl desorption, and thus provide a new carrier doping method that controls the conducting properties of TiNCl.

DOI： 10.1088/1361-648x/abbbc3

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Other Link： https://iopscience.iop.org/article/10.1088/1361-648X/abbbc3/pdf

Publishing type：Research paper (scientific journal)   Publisher：Physical Society of Japan  

DOI： 10.7566/jpsj.89.114707

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Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry (RSC)  

<p>The local structure of BiS₂-based compounds shows an asymmetric Bi-environment with a crucial role of the axial sulfur distance in the electronic transport.</p>

DOI： 10.1039/d0cp03974h

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.carbon.2020.06.025

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Publishing type：Research paper (scientific journal)   Publisher：Wiley  

DOI： 10.1002/pssb.202000117

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1002/pssb.202000117

Publishing type：Research paper (scientific journal)  

© 2020, The Author(s). Skewed band structures have been empirically described in ferroelectric materials to explain the functioning of recently developed ferroelectric tunneling junction (FTJs). Nonvolatile ferroelectric random access memory (FeRAM) and the artificial neural network device based on the FTJ system are rapidly developing. However, because the actual ferroelectric band structure has not been elucidated, precise designing of devices has to be advanced through appropriate heuristics. Here, we perform angle-resolved hard X-ray photoemission spectroscopy of ferroelectric BaTiO3 thin films for the direct observation of ferroelectric band skewing structure as the depth profiles of atomic orbitals. The depth-resolved electronic band structure consists of three depth regions: a potential slope along the electric polarization in the core, the surface and interface exhibiting slight changes. We also demonstrate that the direction of the energy shift is controlled by the polarization reversal. In the ferroelectric skewed band structure, we found that the difference in energy shifts of the atomic orbitals is correlated with the atomic configuration of the soft phonon mode reflecting the Born effective charges. These findings lead to a better understanding of the origin of electric polarization.

DOI： 10.1038/s41598-020-67651-w

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Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/physrevb.101.214526

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.101.214526/fulltext

Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

DOI： 10.1088/1361-648x/ab5e1a

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Other Link： https://iopscience.iop.org/article/10.1088/1361-648X/ab5e1a/pdf

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP Publishing  

Abstract

We have performed soft x-ray spectroscopy in order to study the photoirradiation time dependence of the valence band structure and chemical states of layered transition metal nitride chloride TiNCl. Under the soft x-ray irradiation, the intensities of the states near the Fermi level (E_F) and the Ti³⁺ component increased, while the Cl 2p intensity decreased. Ti 2p–3d resonance photoemission spectroscopy confirmed a distinctive Fermi edge with Ti 3d character. These results indicate the photo-induced metallization originates from deintercalation due to Cl desorption, and thus provide a new carrier doping method that controls the conducting properties of TiNCl.

DOI： 10.1088/1361-648x/abbbc3

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Other Link： https://iopscience.iop.org/article/10.1088/1361-648X/abbbc3/pdf

Language：English   Publishing type：Research paper (scientific journal)  

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Language：English   Publishing type：Research paper (scientific journal)  

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DOI： 10.1088/1361-648X/ab2bd4

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Authorship：Last author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：Physical Society of Japan  

DOI： 10.7566/JPSJ.88.104709

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© 2019 American Physical Society. We systematically investigated the pressure dependence of electrical transport and the crystal structure of topological insulator, Bi2-xSbxTe3-ySey, which showed no superconductivity down to 2.0 K at ambient pressure. The Bi2-xSbxTe3-ySey crystal showed two structural phase transitions under pressure, from rhombohedral structure (space group No. 166, R3m, termed phase I) to monoclinic structure (space group No. 12, C2/m, termed phase II), and from phase II to another monoclinic structure (space group No. 12, C2/m, termed phase III). Superconductivity appeared when applying pressure; actually the superconductivity of all Bi2-xSbxTe3-ySey samples emerged in phase I. The superconducting transition temperature, Tc, increased against pressure in a pressure range of 0-15 GPa for all Bi2-xSbxTe3-ySey samples, and the maximum Tc was 5.45 K, recorded at 13.5 GPa in Bi2-xSbxTe3-ySey at x=0 and y=1.0. The magnetic field (H) dependence of the R-T plot for Bi2-xSbxTe3-ySey was measured to characterize the superconducting pairing mechanism of pressure-induced superconducting phase.

DOI： 10.1103/PhysRevB.100.094525

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acs.nanolett.9b01481

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DOI： 10.1088/1361-6668/ab2792

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

A two-dimensional nanocarbon, graphene, has attracted substantial interest due to its excellent properties. The reduction of graphene oxide (GO) has been investigated for the mass production of graphene used in practical applications. Different reduction processes produce different properties in graphene, affecting the performance of the final materials or devices. Therefore, an understanding of the mechanisms of GO reduction is important for controlling the properties of functional two-dimensional systems. Here, we determined the average structure of reduced GO prepared via heating and photoexcitation and clearly distinguished their reduction mechanisms using Graphene oxide ultrafast time-resolved electron diffraction, time-resolved infrared vibrational spectroscopy, and time-dependent density functional theory calculations. The oxygen atoms of epoxy groups are selectively removed from the basal plane of GO by photoexcitation (photon mode), in stark contrast to the behavior observed for the thermal reduction of hydroxyl and epoxy groups (thermal mode). The difference originates from the selective excitation of epoxy bonds via an electronic transition due to their antibonding character. This work will enable the preparation of the optimum GO for the intended applications and expands the application scope of two-dimensional systems.

DOI： 10.1021/acsnano.9b03060

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DOI： 10.1103/PhysRevB.100.064520

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Institute of Electrical and Electronics Engineers Inc.  

Doping is an important technology for modern science. For example, to create a semiconductor device, a circuit is formed by controlling carriers by doping. It is important to search for appropriate conditions since the carrier emission from dopant differs depending on the doping conditions. The atomic arrangement around the dopant differs depending on the conditions. Therefore, it has been desired to observe the atomic arrangement around the dopant, but it has been difficult with conventional measurement methods. The atomic resolution holography such as photoelectron holography, X-ray fluorescence holography, neutron holography, which are methods that can measure the three-dimensional (3D) atomic arrangement of the dopant. Among them, photoelectron holography can measure the atomic structure of each dopant depending on the chemical state. We have built photoelectron holography apparatuses at BL25SU in SPring-8. We also developed a software platform 3D-AIR-IMAGE for data processing, simulation of photoelectron holograms, and 3D atomic image reconstruction.

DOI： 10.23919/IWJT.2019.8802898

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Authorship：Last author  

DOI： 10.7566/JPSJ.88.041006

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Authorship：Last author  

DOI： 10.1063/1.5087096

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Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

DOI： 10.1038/s41598-019-41906-7

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Other Link： http://www.nature.com/articles/s41598-019-41906-7

Language：English   Publishing type：Part of collection (book)   Publisher：Springer Singapore  

Diamond is a popular material, not only as a gem but also for basic and applied research owing to its extraordinary properties. The discovery of superconductivity in heavily doped diamond has increased its importance. Although the highest superconducting critical temperature (Tc) is 10K, it is regarded as a superconductor with a potentially high Tc, owing to the strong covalent bonding between carbon atoms and the light atomic mass of carbon. In this chapter, after reviewing the fundamentals of the electronic states of pristine and doped diamond, we describe the electronic states, together with the vibrational properties, of borondoped superconductive diamond, which have been revealed by photoemission spectroscopy and other experimental techniques.

DOI： 10.1007/978-981-13-3417-7_3

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Using high-resolution spin-resolved photoemission spectroscopy, we observe a thermal spin depolarization to which all spin-polarized electrons contribute. Furthermore, we observe a distinct minority spin state near the Fermi level and a corresponding depolarization that seldom contributes to demagnetization. The origin of this depolarization has been identified as the many-body effect characteristic of half-metallic ferromagnets. Our investigation opens an experimental field of itinerant ferromagnetic physics focusing on phenomena with sub-meV energy scale.

DOI： 10.1103/PhysRevLett.121.257201

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DOI： 10.1038/s41598-018-36281-8

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DOI： 10.1016/j.jeurceramsoc.2018.07.035

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DOI： 10.1088/1361-648X/aae501

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ROYAL SOC CHEMISTRY  

The local structure of correlated spin-orbit insulator Sr2-xMxIrO4 (M = K, La) has been investigated by Ir L-3-edge extended X-ray absorption fine structure measurements. The measurements were performed as a function of temperature for different dopings induced by substitution of Sr with La or K. It is found that Ir-O bonds have strong covalency and they hardly show any change across the Neel temperature. In the studied doping range, neither Ir-O bonds nor their dynamics, measured by their mean square relative displacements, show any appreciable change upon carrier doping, indicating the possibility of nanoscale phase separation in the doped system. On the other hand, there is a large increase of the static disorder in Ir-Sr correlation, larger for K doping than La doping. Similarities and differences with respect to the local lattice displacements in cuprates are briefly discussed.

DOI： 10.1039/c8cp03756f

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DOI： 10.1103/PhysRevB.98.094525

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Language：Japanese   Publisher：The Japan Society of Applied Physics  

DOI： 10.11470/jsapmeeting.2018.2.0_1334

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Language：Japanese   Publisher：The Japan Society of Applied Physics  

DOI： 10.11470/jsapmeeting.2018.1.0_1434

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Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

DOI： 10.1038/s41598-018-20351-y

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Other Link： http://www.nature.com/articles/s41598-018-20351-y.pdf

Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Synchrotron-based angle-resolved photoemission spectroscopy is used to determine the electronic structure of layered SnSe, which was recently turned out to be a potential thermoelectric material. We observe that the top of the valence band consists of two nearly independent hole bands, whose tops differ by similar to 20meV in energy, indicating the necessity of a multivalley model to describe the thermoelectric properties. The estimated effective masses are anisotropic, with in-plane values of 0.16-0.39 m(0) and an out-of-plane value of 0.71 m(0), where m(0) is the rest electron mass. Information of the electronic structure is essential to further enhance the thermoelectric performance of hole-doped SnSe. (C) 2018 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.57.010301

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

We have studied the local structure of layered Na-x(NH3)(y)MoSe2 system by Mo K-edge extended X-ray absorption fine structure (EXAFS) measurements performed as a function of temperature. We find that molecular intercalation in MoSe2 largely affects the Mo-Se network while Mo-Mo seems to sustain small changes. The Einstein temperature (Theta(E)) of Mo-Mo distance hardly changes (similar to 264 K) indicating that bond strength of this distance remains unaffected by intercalation. On the other hand, Mo-Se distance suffers a softening, revealed by the decrease of (Theta(E)) from similar to 364 K to similar to 350 K. The results indicate that Na+ ion transported by NH3 molecules may enter between the two MoSe-layers resulting reduced Se-Se coupling. Therefore, increased hybridization between Se 4p and Mo 4d orbitals due to inter-layer disorder is the likely reason of metallicity in intercalated MoSe2 and superconductivity at low temperature.

DOI： 10.1016/j.jpcs.2017.07.011

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We have studied the local structure of superconducting Ca10Pt4As8(Fe2As2)(5) (Pt10418) and Ca10Ir4As8(Fe2As2)(5) (Ir10418) iron arsenides, showing different transition temperatures (T-c = 38 and 16 K, respectively), by polarized Fe K-edge extended x-ray absorption fine-structure measurements. Despite the similar average crystal structures, the local structures of the FeAs4 tetrahedra in the two compounds are found to be very different. The FeAs4 in Pt10418 is close to a regular tetrahedron, while it deviates largely in Ir10418. The Fe-Fe correlations in the two compounds are characterized by similar bond-length characteristics; however, the static disorder in Pt10418 is significantly lower than that in Ir10418. The results suggest that the optimized local structure and reduced disorder are the reasons for higher Tc and well-defined electronic states in Pt10418 unlike Ir10418 showing the coexistence of glassy and normal electrons at the Fermi surface, and hence provide direct evidence of the local-structure-driven optimization of the electronic structure and superconductivity in iron arsenides.

DOI： 10.1103/PhysRevB.96.224507

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ROYAL SOC CHEMISTRY  

We have studied the valence electronic structure of Ag1-xSn1+xSe2 (x = 0.0, 0.1, 0.2, 0.25) and SnSe (x = 1.0) by a combined analysis of X-ray absorption spectroscopy (XAS) and X-ray photoemission spectroscopy (XPS) measurements. Both XAS and XPS reveal an increase in electron carriers in the system with x (i.e. excess Sn concentration) for 0 &lt;= x &lt;= 0.25. The core-level spectra (Sn 3d, Ag 3d and Se 3d) show that the charge state of Ag is almost 1+, while that of of Sn splits into Sn2+ and Sn4+ (providing clear evidence of valence skipping for the first time) with a concomitant splitting of Se into Se2- and Se2-delta states. The x dependence of the split components in Sn and Se together with the Se-K edge XAS reveals that the Se valence state may have an essential role in the transport properties of this system.

DOI： 10.1039/c7cp05369j

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We have investigated the electronic states and spin polarization of half-metallic ferromagnet CrO2 (100) epitaxial films by bulk-sensitive spin-resolved photoemission spectroscopy with a focus on non-quasiparticle (NQP) states derived from electron-magnon interactions. We found that the averaged values of the spin polarization are approximately 100% and 40% at 40 K and 300 K, respectively. This is consistent with the previously reported result [H. Fujiwara et al., Appl. Phys. Lett. 106, 202404 (2015).]. At 100 K, peculiar spin depolarization was observed at the Fermi level (E-F), which is supported by theoretical calculations predicting NQP states. This suggests the possible appearance of NQP states in CrO2. We also compare the temperature dependence of our spin polarizations with that of the magnetization. (C) 2017 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.elspec.2017.02.003

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We have performed scanning photoemission spectromicroscopy of BaV10O15 across the metal-insulator transition at 123 K, which is accompanied by V 3d charge/orbital order and V trimerization. Nucleation of metallic domains is observed at the cleaved surface of BaV10O15 single crystals, similar to Cr-doped V2O3 in which electronic configurations of Cr3+ and V3+ are the same as those of V2+ and V3+ in BaV10O15. Typical domain size is similar to 5- 10 mu m at 150 K, just above the transition temperature. The metallic domains continuously grow up to 240 K, well above the transition temperature. The temperature evolution of the metallic phase in BaV10O15 is different from that of Cr-doped V2O3, probably due to the charge degrees of freedom in BaV10O15.

DOI： 10.1103/PhysRevB.96.115161

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Language：Japanese   Publisher：The Japan Society of Applied Physics  

DOI： 10.11470/jsapmeeting.2017.2.0_1242

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCI LTD  

The effects of aliovalent dopants on the kinetics of spinodal decomposition in a rutile-type TiO2-VO2 system were studied by means of x-ray diffraction and transmission electron microscopy. In this study, as prepared polycrystalline samples were annealed at 673 k inside the spinodal. For undoped samples, phase decomposition occurred along the [001] direction, and a lamellar structure formed on the nanometer length scale. The phase decomposition kinetics in this system were sensitive to the type of aliovalent dopant; the decomposition rate in a sample doped with Al2O3 was more than one order of magnitude higher than a sample doped with . These observations could be explained on the basis of the premise that cation interstitial mobility is greater than cation vacancy mobility. The obtained results are consistent with those for a TiO2-SnO2 system, indicating that phase separation in both rutile-type oxide systems can be interpreted through a common framework. (C) 2017 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jeurceramsoc.2017.03.039

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The electronic structure of the lightly electron-doped correlated spin-orbit insulator Sr2IrO4 has been studied by angle-resolved photoelectron spectroscopy. We have observed the coexistence of a lower Hubbard band and an in-gap band; the momentum dependence of the latter traces that of the band calculations without on-site Coulomb repulsion. The in-gap state remained anisotropically gapped in all observed momentum areas, forming a remnant Fermi-surface state, evolving towards the Fermi energy by carrier doping. These experimental results show a striking similarity with those observed in deeply underdoped cuprates, suggesting the common nature of the nodal liquid states observed in both compounds.

DOI： 10.1103/PhysRevB.96.041106

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society  

We investigate superconductivity in the candidate topological superconductor NbxBi2Se3 from x=0 to 1. We observe superconductivity for all x&gt
0 with a superconducting transition temperature TC∼ 3 K similar to that reported in Cu- and Sr-intercalated Bi2Se3. Structurally, for small x we find introduction of Nb modifies the Bi2Se3 separation while for larger x the system is driven to the BiSe structure. While all compounds are found to be superconducting, two distinct behaviors for the upper critical field Hc2 are observed. From photoemission spectroscopy we find that the enhanced Hc2 corresponds to compounds with a low surface Dirac point energy (-0.67 eV) relative to the Fermi level
in absolute scale this value is lower than previously reported superconductors in this materials family. We comment on the utility of this system for the study of topological superconductivity in a new structure type combining a doping (Bi2 type) layer and layers with topologically protected gapless state (Bi2Se3 type).

DOI： 10.1103/PhysRevB.95.180503

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We have performed potassium (K)-doping dependent photoemission (PES) measurements of picene film on highly oriented pyrolytic graphite (HOPG). K-doping dependent valence band PES data exhibits a shift of the valence band to a higher binding energy, indicative of the charge transfer from the K atoms to the picene film. K-doping dependent PES spectra near the Fermi level (EF) show the appearance and disappearance of a Fermi edge, indicating the metallic properties of the film at certain K concentrations. High-resolution PES spectrum at the lowest measured temperature does not show an opening of the superconducting gap, requesting further studies to explore the superconducting properties of the K-doped picene film. The results will be discussed by comparison with previous and recent spectroscopic studies.

DOI： 10.1088/1361-648X/29/6/064001

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We have studied the effect of Se substitution on Eu valence in a layered Eu0.5La0.5FBiS2-x Se-x superconductor using a combined analysis of x-ray absorption near-edge structure (XANES)and x-ray photoelectron spectroscopy (XPS) measurements. Eu L-3-edge X Lambda NES spectra reveal that Eu is in the mixed valence state with coexisting Eu2+ and Eu3+. The average Eu valence decreases sharply from similar to 2.3 for x = 0.0 to similar to 2.1 for x = 0.4. Consistently, Eu 3d XPS shows a clear decrease in the average valence by Se substitution. Bi 4f XPS indicates that effective charge carriers in the BiCh(2) (Ch = S, Se) layers are slightly increased by Se substitution. On the basis of the present results it has been discussed that the metallic character induced by Se substitution in Eu0.5La0.5FBiS2- x Se-x is likely to be due to increased in-plane orbital overlap driven by reduced in-plane disorder that affects the carrier mobility.

DOI： 10.1103/PhysRevB.95.064515

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© 2017 American Physical Society.We have studied the electronic structure of Eu3F4Bi2S4 using a combination of Eu L3-edge x-ray absorption spectroscopy (XAS) and space-resolved angle-resolved photoemission spectroscopy (ARPES). From the Eu L3-edge XAS, we have found that the Eu in this system is in mixed valence state with coexistence of Eu2+/Eu3+. The bulk charge doping was estimated to be ∼0.3 per Bi site in Eu3F4Bi2S4, which corresponds to the nominal x in a typical REO1-xFxBiS2 system (RE: rare-earth elements). From the space-resolved ARPES, we have ruled out the possibility of any microscale phase separation of Eu valence in the system. Using a microfocused beam we have observed the band structure as well as the Fermi surface that appeared similar to other compounds of this family with disconnected rectangular electronlike pockets around the X point. The Luttinger volume analysis gives the effective carrier to be 0.23 electrons per Bi site in Eu3F4Bi2S4, indicating that the system is likely to be in the underdoped region of its superconducting phase diagram.

DOI： 10.1103/PhysRevB.95.035152

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We have studied the electronic structure of Eu3F4Bi2S4 using a combination of Eu L-3-edge x-ray absorption spectroscopy (XAS) and space-resolved angle-resolved photoemission spectroscopy (ARPES). From the Eu L-3-edge XAS, we have found that the Eu in this system is in mixed valence state with coexistence of Eu2+/Eu3+. The bulk charge doping was estimated to be similar to 0.3 per Bi site in Eu3F4Bi2S4, which corresponds to the nominal chi in a typical REO1-chi F chi BiS2 system (RE: rare-earth elements). From the space-resolved ARPES, we have ruled out the possibility of any microscale phase separation of Eu valence in the system. Using a microfocused beam we have observed the band structure as well as the Fermi surface that appeared similar to other compounds of this family with disconnected rectangular electronlike pockets around the X point. The Luttinger volume analysis gives the effective carrier to be 0.23 electrons per Bi site in Eu3F4Bi2S4, indicating that the system is likely to be in the underdoped region of its superconducting phase diagram.

DOI： 10.1103/PhysRevB.95.035152

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DOI： 10.1103/PhysRevB.95.085109

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ROYAL SOC CHEMISTRY  

We have studied the local structure and valence electronic unoccupied states of thermoelectric CsBi4Te6 and superconducting CsBi3.5Pb0.5Te6 (T-c similar to 3 K) by extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) measurements. The Bi-L-3 edge EXAFS reveals wide Bi-Te distance distribution for both compounds indicating complex atomic arrangements in the studied system. The mean square relative displacements (MSRDs) of the Bi-Te bond distances appear largely increased in Pb substituted system due to larger overall local disorder, however, one of the Bi-Te bonds shows a reduced disorder. On the other hand, the Bi-L-3 edge XANES is hardly affected by Pb substitution while the Te-L-1 edge XANES reveals increased density of unoccupied Te 5p states. This suggests that the carriers introduced by the Pb substitution in CsBi4-xPbxTe6 preferentially goes on Te sites. Similarly, the Cs-L-3 edge XANES also shows small changes due to Pb-substitution and reduced local disorder indicated by the reduced width of the Cs-L3 edge white line. We have also shown that the X-ray photoemission spectroscopy (XPS) measurements on various electronic core levels are in a qualitative agreement with the XANES results. These findings are consistent with carrier doping and a reduced disorder in one direction to be likely factors to drive the thermoelectric CsBi4Te6 into a bulk superconductor by Pb-substitution in CsBi4-xPbxTe6.

DOI： 10.1039/c6cp04949d

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We report syntheses of new superconducting metal-doped MoSe2 materials (MxMoSe2). The superconducting MxMoSe2 samples were prepared using a liquid NH3 technique, and can be represented as '(NH3)(y)MxMoSe2'. The T(c)s of these materials were approximately 5.0 K, independent of x and the specific metal atom. X-ray diffraction patterns of (NH3)(y)NaxMoSe2 were recorded using polycrystalline powders. An increase in lattice constant c showed that the Na atom was intercalated between MoSe2 layers. The x-independence of c was observed in (NH3)(y)NaxMoSe2, indicating the formation of a stoichiometric compound in the entire x range, which is consistent with the x-independence of T-c. A metallic edge of the Fermi level was observed in the photoemission spectrum at 30 K, demonstrating its metallic character in the normal state. Doping of MoSe2 with Li and K also yielded superconductivity. Thus, MoSe2 is a promising material for designing new superconductors, as are other transition metal dichalcogenides.

DOI： 10.1038/srep29292

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

One of the ultimate goals of the study of iron-based superconductors is to identify the common feature that produces the high critical temperature (T-c). In the early days, based on a weak-coupling viewpoint, the nesting between hole-and electron-like Fermi surfaces (FSs) leading to the so-called s +/- state was considered to be one such key feature. However, this theory has faced a serious challenge ever since the discovery of alkali-metal-doped FeSe (AFS) superconductors, in which only electron-like FSs with a nodeless superconducting gap are observed. Several theories have been proposed, but a consistent understanding is yet to be achieved. Here we show experimentally that a hole-like band exists in KxFe2-ySe2, which presumably forms a hole-like Fermi surface. The present study suggests that AFS can be categorized in the same group as iron arsenides with both hole- and electron-like FSs present. This result provides a foundation for a comprehensive understanding of the superconductivity in iron-based superconductors.

DOI： 10.7566/JPSJ.85.073704

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We have made Cr-53 NMR measurements on polycrystalline and single-crystalline film samples to study the local electronic state of a half-metallic ferromagnet CrO2 which has the rutile structure with one crystallographically equivalent Cr site. We observe two kinds of Cr-53 NMR spectra in the ferromagnetic state of both the samples. An analysis on the hyperfine field shows the presence of two Cr sites with different orbital occupancies, suggesting that a local orbital order takes place with breaking the local symmetry in the rutile structure. This may be ascribed to the negative charge transfer between chromium and oxygen ions.

DOI： 10.1103/PhysRevB.93.235129

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Recently, ammonia-thermal reaction has been used for molecular intercalation in layered FeSe, resulting a new Li-x(NH3)(y)Fe2Se2 superconductor with T-c similar to 45 K. Here, we have used temperature dependent extended x-ray absorption fine structure (EXAFS) to investigate local atomic displacements in single crystals of this new superconductor. Using polarized EXAFS at Fe K-edge we have obtained direct information on the local Fe-Se and Fe-Fe bondlengths and corresponding mean square relative displacements (MSRD). We find that the Se-height in the intercalated system is lower than the one in the binary FeSe, suggesting compressed FeSe4 tetrahedron in the title system. Incidentally, there is hardly any effect of the intercalation on the bondlengths characteristics, revealed by the Einstein temperatures, that are similar to those found in the binary FeSe. Therefore, the molecular intercalation induces an effective compression and decouples the FeSe slabs. Furthermore, the results reveal an anomalous change in the atomic correlations across T-c, appearing as a clear decrease in the MSRD, indicating hardening of the local lattice mode. Similar response of the local lattice has been found in other families of superconductors, e.g., A15-type and cuprates superconductors. This observation suggests that local atomic correlations should have some direct correlation with the superconductivity.

DOI： 10.1038/srep27646

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE SA  

TaO2 thin films were prepared using rutile-type NbO2 template layers on Al2O3 (0001) substrates by a pulsed laser deposition technique. The structural and electronic properties of the films were characterized by performing four-circle X-ray diffraction (XRD) and X-ray photoemission spectroscopy (XPS) measurements. Four-circle XRD measurements revealed the formation and epitaxial growth of (100)-oriented tetragonal TaO2 on the NbO2 template layers. On the other hand, XPS measurements showed two kinds of valence states, namely Ta4+ and Ta5+ in the surface region of the film. The results from the XRD and XPS measurements suggest that the film is composed of epitaxially grown TaO2 with an amorphous Ta2O5 surface layer. It is found from control experiments that the NbO2 template layers play an important role in inducing the crystallization of the TaO2 films and promoting the formation of Ta4+ in the films deposited thereon. The results indicate the effectiveness of the NbO2 template layers for the formation of TaO2 thin films. Our work presents a promising method for the preparation of TaO2 thin films and provides a starting point for further research on TaO2. (C) 2015 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.tsf.2015.12.066

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

BiS2-based layered superconductors are new superconductors, having been paid special attention due to the layered crystal structure and the characteristic electronic structure that can lead to exotic mechanism of superconductivity as well as cooperation of superconductivity and ordered phases. In this article, we report valence band dispersions and Fermi surface topology of LaO1-xFxBiS2 (x = 0.23, T-c &lt; 2 K) studied by angle-resolved photoemission spectroscopy, and compared them with those of LaO1-xFxBiS2 (x = 0.46, T-c = 3.1 K). We discuss experimental x dependent electronic structures and its relation to T-c. We also point to anomalous spectroscopic features in LaO1-xFxBiS2 (x = 0.46) that can be explained by band structure calculations considering distorted lattice, suggesting role of dynamical or static lattice distortion to the physical properties.

DOI： 10.1088/1742-6596/683/1/012002

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

Bulk electronic structure of novel layered superconductor Nd(O,F)BiS2 was studied by using soft x-ray angle-resolved photoelectron spectroscopy (ARPES). Electron-like Fermi surface centered at the X(R) point was observed, consistent with earlier ARPES reports on surface-sensitive VUV light source. Based on the comparison of the electronic structure between Nd(O,F)BiS2 and La(O,F)BiS2, we discuss possible important factors for the superconductivity in this series of material.

DOI： 10.1088/1742-6596/683/1/012003

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

DOI： 10.11316/jpsgaiyo.71.1.0_1441

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DOI： 10.11316/jpsgaiyo.71.1.0_2492

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DOI： 10.11316/jpsgaiyo.71.1.0_2315

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'Aromatic hydrocarbon' implies an organic molecule that satisfies the (4n + 2) pi-electron rule and consists of benzene rings. Doping solid aromatic hydrocarbons with metals provides the superconductivity. The first discovery of such superconductivity was made for K-doped picene (K(x)picene, five benzene rings). Its superconducting transition temperatures (T-c's) were 7 and 18 K. Recently, we found a new superconducting K(x)picene phase with a T-c as high as 14 K, so we now know that K(x)picene possesses multiple superconducting phases. Besides K(x)picene, we discovered new superconductors such as Rb(x)picene and Caxpicene. A most serious problem is that the shielding fraction is &lt;= 615% for K(x)picene and Rb(x)picene, and it is often similar to 1% for other superconductors. Such low shielding fractions have made it difficult to determine the crystal structures of superconducting phases. Nevertheless, many research groups have expended a great deal of effort to make high quality hydrocarbon superconductors in the five years since the discovery of hydrocarbon superconductivity. At the present stage, superconductivity is observed in certain metaldoped aromatic hydrocarbons (picene, phenanthrene and dibenzopentacene), but the shielding fraction remains stubbornly low. The highest priority research area is to prepare aromatic superconductors with a high superconducting volume-fraction. Despite these difficulties, aromatic superconductivity is still a core research target and presents interesting and potentially breakthrough challenges, such as the positive pressure dependence of T-c that is clearly observed in some phases of aromatic hydrocarbon superconductors, suggesting behavior not explained by the standard BCS picture of superconductivity. In this article, we describe the present status of this research field, and discuss its future prospects. (C) 2015 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2015.02.015

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We studied the electronic structure of Ca-10(Pt4As8)(Fe2-xPtxAs2)(5) for two platinum concentrations with different T-c using ARPES. From the carrier number estimated from the areas of FS sheets, we found that the Pt substitution induces electron doping into the Fe2As2 layers (effectively two electrons per Pt atom). In addition, we found that the FS nesting condition in x = 0.25 is stronger than that in x = 0.42, indicating close correlation between the FS nesting and T-c.

DOI： 10.7566/JPSJ.84.055001

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We have performed bulk-sensitive spin-resolved photoemission spectroscopy in order to clarify the intrinsic spin-resolved electronic states of half-metallic ferromagnet CrO2. We used CrO2 epitaxial films on TiO2(100), which shows a peak at 1 eV with a clear Fermi edge, consistent with the bulk-sensitive PES spectrum for CrO2. In spin-resolved spectra at 40 K, while the Fermi edge was observed in the spin up (majority spin) state, no states at the Fermi level (EF) with an energy gap of 0.5 eV below EF were observed in the spin down (minority spin) state. At 300 K, the gap in the spin down state closes. These results are consistent with resistivity measurements and magnetic hysteresis curves of the fabricated CrO2 film, constituting spectroscopic evidence for the half-metallicity of CrO2 at low temperature and reducing the spin polarization at room temperature. We also discuss the electron correlation effects of Cr 3d. (C) 2015 AIP Publishing LLC.

DOI： 10.1063/1.4921629

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We have observed zero resistivity above 10K and an onset of resistivity reduction at 25.2K in a heavily B-doped diamond film. However, the effective carrier concentration is similar to that of superconducting diamond with a lower T-c. We found that the carrier has a longer mean free path and lifetime than in the previous report, indicating that this highest T-c diamond has better crystallinity compared to that of other superconducting diamond films. In addition, the susceptibility shows a small transition above 20K in the high quality diamond, suggesting a signature of superconductivity above 20 K. These results strongly suggest that heavier carrier doped defect-free crystalline diamond could give rise to high T-c diamond. (C) 2015 AIP Publishing LLC.

DOI： 10.1063/1.4907411

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

DOI： 10.11316/jpsgaiyo.70.2.0_1681

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DOI： 10.11316/jpsgaiyo.70.2.0_942

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DOI： 10.11316/jpsgaiyo.70.2.0_1720

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DOI： 10.11316/jpsgaiyo.70.2.0_2405

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DOI： 10.11316/jpsgaiyo.70.2.0_1933

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Language：Japanese   Publisher：一般社団法人日本物理学会  

DOI： 10.11316/jpsgaiyo.70.2.0_1237

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DOI： 10.1103/PhysRevB.90.220512

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The electronic structure of nearly optimally doped novel superconductor LaO1-xFxBiS2(x = 0.46) was investigated using angle-resolved photoemission spectroscopy (ARPES). We clearly observed band dispersions from 2 to 6 eV binding energy and near the Fermi level (E-F), which are well reproduced by first-principles calculations when the spin-orbit coupling is taken into account. The ARPES intensity map near E-F shows a squarelike distribution around the Gamma(Z) point in addition to electronlike Fermi-surface (FS) sheets around the X(R) point, indicating that FS of LaO0.54F0.46BiS2 is in close proximity to the theoretically predicted topological change.

DOI： 10.1103/PhysRevB.90.220512

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The transport and magnetic properties of the tetragonal Fe1+delta S were investigated using magnetoresistivity and magnetization within 2 &lt;= T &lt;= 300 K, H &lt;= 70 kOe and P &lt;= 3.0 GPa. In addition, room-temperature x-ray diffraction and photoelectron spectroscopy were also applied. In contrast to previously reported nonmetallic character, Fe1+delta S is intrinsically metallic but due to a presence of a weak localization such metallic character is not exhibited below room temperature. An applied pressure reduces strongly this additional resistive contribution and as such enhances the temperature range of the metallic character which, for similar to 3 GPa, is evident down to 75 K. The absence of superconductivity as well as the mechanism behind the weak localization will be discussed.

DOI： 10.1088/1468-6996/15/5/055007

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We have performed a photoemission spectroscopy (PES) study of CeM2Al10 (M = Fe, Ru, and Os) to directly observe the electronic structure involved in the unusual magnetic ordering. Soft X-ray resonant (SXR) PES provides spectroscopic evidence of the hybridization between conduction and Ce 4f electrons (c-f hybridization) and the order of the hybridization strength (Ru &lt; Os &lt; Fe). High-resolution (HR) PES of CeRu2Al10 and CeOs2Al10, as compared with that of CeFe2Al10, identifies two structures that can be ascribed to structures induced by the c-f hybridization and the antiferromagnetic ordering, respectively. Although the c-f hybridization-induced structure is a depletion of the spectral intensity (pseudogap) around the Fermi level (E-F) with an energy scale of 20-30 meV, the structure related to the antiferromagnetic ordering is observed as a shoulder at approximately 10-11 meV within the pseudogap. The energies of the shoulder structures of CeRu2Al10 and CeOs2Al10 are approximately half of the optical gap (20 meV), indicating that E-F is located at the midpoint of the gap.

DOI： 10.7566/JPSJ.83.094717

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We present a persistent soft-X-ray-induced insulator-to-metal transition at room temperature for VO2 thin films. Continuous soft X-ray irradiation renders the insulating electronic state metallic in the VO2 thin films, which has been found by photoemission spectroscopy measurement. The observed transition is irreversible and the metallic state persists when soft X-ray irradiation is stopped. The analysis of valence band spectra reveals that the density of states (DOS) of the V 3d band increases with irradiation time, while the DOS of the O 2p band decreases. We propose a simple model where the persistent insulator-to-metal transition is driven by oxygen desorption from VO2 thin films under soft X-ray irradiation. The present results will help in developing a new route for controlling the phase transition of VO2 by light. (C) 2014 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.53.05FB09

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We use core level and valence band soft X-ray photoemission spectroscopy (SXPES) to investigate the electronic structure of new BiS2 layered superconductor LaO1-xFxBiS2. The core level spectra of doped samples show a new spectral feature on the lower binding energy side of the Bi 4 f main peak, which may be explained by core-hole screening with metallic states near the Fermi level (E-F). The experimental electronic structure and its x dependence (a higher binding energy shift of the valence band as well as appearance of new states near E-F having the dominant Bi 6p character) were found to be consistent with the predictions of band structure calculations in general. Noticeable deviation of the spectral shape of the states near EF from that of calculations might provide insight into interesting physical properties. This study provides the first experimental electronic structure of the new BiS2 layered superconductors.

DOI： 10.7566/JPSJ.83.033703

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

Unconventional Cooper pairing originating from spin or orbital fluctuations has been proposed for iron-based superconductors. Such pairing may be enhanced by quasi-nesting of two-dimensional electron and hole-like Fermi surfaces (FS), which is considered an important ingredient for superconductivity at high critical temperatures (high-T-c). However, the dimensionality of the FS varies for hole and electron-doped systems, so the precise importance of this feature for high-T-c materials remains unclear. Here we demonstrate a phase of electron-doped CaFe2As2 (La and P co-doped CaFe2As2) with T-c = 45 K, which is the highest T-c found for the AEFe(2)As(2) bulk superconductors (122-type; AE = Alkaline Earth), possesses only cylindrical hole-and electron-like FSs. This result indicates that FS topology consisting only of two-dimensional sheets is characteristic of both hole-and electron-doped 122-type high-T-c superconductors.

DOI： 10.1038/srep04381

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DOI： 10.1103/PhysRevB.88.245414

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We report the results of photoemission spectroscopy on the potassium concentration (x) dependence of K-x picene films on graphite. The results show a systematic shift of a whole valence band and the appearance of a clear Fermi edge for a certain value of x, with additional structures at higher binding energies. Observed spectral evolutions are reminiscent to those of doped C-60, implying that the K-x picene film has multiple phases depending on the K concentration. Appearance of a clear Fermi edge, in contrast to multilayer K-x picene films on Au, SiO2, and Ag(111) substrates, where no clear Fermi edges are observed, suggests that the emergence of the metallic states for K-x picene films depends on the specific molecular orientations in the films on different substrates.

DOI： 10.1103/PhysRevB.88.245414

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We have investigated the changes in the band structure and core-level energy of Ca(Fe1-xRhx)(2)As-2 across the tetragonal-collapsed tetragonal (T-cT) phase transition using photoemission spectroscopy. Angle-resolved photoemission spectroscopy clearly demonstrates that, at the Gamma point in the Brillouin zone, a hole-like band that crosses the Fermi level in the T phase lowers its energy position and is completely filled in the cT phase. This indicates the disappearance of a hole-like Fermi surface around the zone center in the cT phase, which gives rise to the extinction of nesting. The relationship of the present results with the results of previous transport and magnetic studies is discussed. The temperature dependences of the As 3d and Ca 2p core levels show energy shifts toward higher and lower binding energies, respectively. These data constitute the first direct experimental results on electronic structure and chemical states across the T-cT transition.

DOI： 10.7566/JPSJ.82.073705

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The magnetocapacitance effect was investigated using impedance spectroscopy on single crystals of LuFe2O4. The intrinsic impedance response could be separated from the interfacial response and showed a clear hysteresis loop below T-Ferri similar to 240 K under the magnetic field. The neutron diffraction experiment under the magnetic field proves the origin of the dielectric property related to the motion of the nanosized ferromagnetic domain boundary. These results imply that the modification of the microscopic domain structure is responsible for the magnetoelectric effect in LuFe2O4. DOI: 10.1103/PhysRevLett.110.117602

DOI： 10.1103/PhysRevLett.110.117602

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URu 2 Si 2 is a uranium compound that exhibits a so-called 'hidden-order' transition at ∼17.5â€...K. However, the order parameter of this second-order transition as well as many of its microscopic properties remain unclarified despite considerable research. One of the key questions in this regard concerns the type of spontaneous symmetry breaking occurring at the transition
although rotational symmetry breaking has been detected, it is not clear whether another type of symmetry breaking also occurs. Another key question concerns the property of Fermi-surface gapping in the momentum space. Here we address these key questions by a momentum-dependent observation of electronic states at the transition employing ultrahigh-resolution three-dimensional angle-resolved photoemission spectroscopy. Our results provide compelling evidence of the spontaneous breaking of the lattice's translational symmetry and particle-hole asymmetric gapping of a heavy quasiparticle pocket at the transition.

DOI： 10.1038/srep02750

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATL INST MATERIALS SCIENCE  

We have characterized the electronic structure of FeSe1-xTex for various x values using soft x-ray photoemission spectroscopy (SXPES), high-resolution photoemission spectroscopy (HRPES) and inverse photoemission spectroscopy (IPES). The SXPES valence band spectral shape shows that the 2 eV feature in FeSe, which was ascribed to the lower Hubbard band in previous theoretical studies, becomes less prominent with increasing x. HRPES exhibits systematic x dependence of the structure near the Fermi level (E-F): its splitting near EF and filling of the pseudogap in FeSe. IPES shows two features, near E-F and approximately 6 eV above E-F; the former may be related to the Fe 3d states hybridized with chalcogenide p states, while the latter may consist of plane-wave-like and Se d components. In the incident electron energy dependence of IPES, the density of states near E-F for FeSe and FeTe has the Fano lineshape characteristic of resonant behavior. These compounds exhibit different resonance profiles, which may reflect the differences in their electronic structures. By combining the PES and IPES data the on-site Coulomb energy was estimated at 3.5 eV for FeSe.

DOI： 10.1088/1468-6996/13/5/054403

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We performed a comparative study of the superconducting gap in the new filled skutterudite superconductors LaPt4Ge12 and PrPt4Ge12 using high-resolution photoemission spectroscopy. We succeeded in observing spectral changes across T-c that reflect the opening of the superconducting gap in both compounds and also in observing a noticeable difference in their respective superconducting spectral shapes near the Fermi level, pointing toward a more complex superconducting gap structure in PrPt4Ge12. In addition, we found that the two-gap model is more suitable for describing the superconducting-state spectrum of PrPt4Ge12 than the single-isotropic-gap and single-anisotropic-gap models, which suggests an explanation that multiband effects may possibly induce the anomalous superconducting properties of PrPt4Ge12.

DOI： 10.1103/PhysRevB.86.014521

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We studied the spectral functions of URu2Si2 in the hidden-order state by means of ultrahigh-resolution angle-resolved photoemission spectroscopy. High signal-to-noise ratio data uncover the existence of two anomalous fine structures, namely a "splitting" in the previously reported narrow dispersive band, and a small, dispersionless "satellite" structure. These structures can be explained by neither ordinary bands nor the development of a hybridization of two states; thus our observation should evoke the consideration of complicated many-body effects. As a possible origin, we suggest the existence of electron-mode coupling in the ordered state.

DOI： 10.1103/PhysRevB.85.241102

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DOI： 10.1103/PhysRevB.85.241102

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We have investigated the effectiveness of a hot-filament chemical vapor deposition (HFCVD) method for preparation of a boron-doped superconducting diamond film with higher superconducting transition temperature (T-c). A boron-doped superconducting diamond film has been fabricated on a diamond (111) substrate using the HFCVD method, and studied by means of scanning electron microscopy. X-ray photoelectron spectroscopy (XPS), and resistivity. The film consists of grains with an average size of 200 mm. Analyses of valence band and boron Is core-level XPS spectra indicate the formation of boron-doped diamond film on the substrate. From the resistivity measurements, the film is found to be a superconductor with onset T-c of 7.1 K. Carrier concentration determined by Hall conductivity measurements is 1.1 x 10(21) cm(-3). The value of T-c is higher compared with that in boron-doped superconducting diamond films prepared by a commonly used microwave plasma-assisted chemical vapor deposition (MPCVD) method, at the same carrier concentration [A. Kawano et al., Phys. Rev. B 82 (2010) 085318]. The result of higher T-c in the film by the HFCVD method is consistent with the previous one [Wang et al., Diamond Relat. Mater. 15 (2006) 659], suggesting that the HFCVD method is effective for preparation of boron-doped superconducting diamond films showing higher T-c. (C) 2012 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.diamond.2012.02.008

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-BLACKWELL  

A system for angle-resolved photoemission spectroscopy (ARPES) of small single crystals with sizes down to 100 mm has been developed. Soft X-ray synchrotron radiation with a spot size of similar to 40 mu m x 65 mu m at the sample position is used for the excitation. Using this system an ARPES measurement has been performed on a Si crystal of size 120 mu m x 100 mu m x 80 mu m. The crystal was properly oriented on a sample stage by measuring the Laue spots. The crystal was cleaved in situ with a microcleaver at 100 K. The cleaved surface was adjusted to the beam spot using an optical microscope. Consequently, clear band dispersions along the Gamma-X direction reflecting the bulk electronic states were observed with a photon energy of 879 eV.

DOI： 10.1107/S0909049511034418

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We have investigated the electronic structures of Rh17S15 by using soft x-ray photoemission spectroscopy and first principle band structure calculations. While S 2p core level spectra are explained by symmetric components, Rh 3d core level exhibits asymmetric line shape, suggesting a dominant Rh 4d character at the states near the Fermi level. The binding energy of Rh 4d in Rh17S15 is similar to the one in Rh metal, and this observation implies metallic Rh-Rh bonding. Many features in experimental valence spectra are reasonably described by the calculations, indicating that one particle approximation is a good starting point to understand the electronic properties of Rh17S15. The comparison between the experiment and the calculation suggests that Rh 4d character at the states near the Fermi level is essential for the correlated superconductivity in Rh17S15. However, some deviation between the experiment and the calculation as well as a finite mass enhancement are found, indicating some corrections on the treatment of electron-phonon coupling and/or electron correlations are necessary.

DOI： 10.1143/JPSJS.80SA.SA111

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We studied the electronic structure evolution of heavily B-doped diamond films across the metal-insulator transition (MIT) using ultraviolet photoemission spectroscopy (UPS). From high-temperature UPS, through which electronic states near the Fermi level (E(F)) up to similar to 5k(B)T can be observed (k(B) is the Boltzmann constant and T the temperature), we observed the carrier concentration dependence of spectral shapes near E(F). Using another carrier concentration dependent UPS, we found that the change in energy position of sp-band of the diamond valence band, which corresponds to the shift of E(F), can be explained by the degenerate semiconductor model, indicating that the diamond valence band is responsible for the metallic states for samples with concentrations above MIT. We discuss a possible electronic structure evolution across MIT. (C) 2010 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jpcs.2010.10.052

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

We have studied the electronic structures of URu2Si2 employing ultrahigh-resolution laser angle-resolved photoemission spectroscopy. The change of photoemission spectra is investigated across the hidden-order transition, and the emergence of a narrow band is clearly observed near the Fermi level for both (pi,0) and (pi,pi) directions. In addition, it is shown that tuning of light&apos;s polarization allows the signal of a hole-like dispersive feature to enhance. These observations prove that laser angle-resolved photoemission spectroscopy is an effective tool for studying the evolution of electronic structures across the hidden-order transition in URu2Si2. (C) 2010 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jpcs.2010.10.048

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We have prepared VO2 thin films epitaxially grown on TiO2(001) substrates with thickness systematically varied from 2.5 to 13 nm using a pulsed laser deposition method, and studied the transport property and electronic states of the films by means of resistivity and in situ synchrotron photoemission spectroscopy (SRPES). In resistivity measurements, the 13-nm-thick film exhibits a metal-insulator transition at around 290 K on cooling with change of three orders of magnitudes in resistivity. As the film thickness decreases, the metal-insulator transition broadens and the transition temperature increases. Below 4 nm, the films do not show the transition and become insulators. In situ SRPES measurements of near the Fermi level valence band find that the electronic state of the 2.5-nm-thick film is different than that of the temperature-induced insulator phase of VO2 itself although these two states are insulating. Ti 2p core-level photoemission measurements reveal that Ti ions exist near the interface between the films and TiO2 substrates, with a chemical state similar to that in (V,Ti)O-2 solid solution. These results indicate that insulating (V,Ti)O-2 solid solution is formed in the thinner films. We propose a simple growth model of a VO2 thin film on a TiO2(001) substrate. Near the interface, insulating (V,Ti) O-2 solid solution is formed due to the diffusion of Ti ions from the TiO2 substrate into the VO2 film. The concentration of Ti in (V,Ti) O-2 is relatively high near the interface and decreases toward the surface of the film. Beyond a certain film thickness (about 7 nm in the case of the present 13-nm-thick film), the VO2 thin film without any Ti ions starts to grow. Our work suggests that developing a technique for preparing the sharp interface between the VO2 thin films and TiO2 substrates is a key issue to study the physical property of an ultrathin film of "pure" VO2, especially to examine the presence of the novel electronic state called a semi-Dirac point phase predicted by calculations. (c) 2011 American Institute of Physics. [doi:10.1063/1.3549835]

DOI： 10.1063/1.3549835

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We have performed high-resolution core level photoemission spectroscopy on a heavily phosphorus (P)-doped diamond film in order to elucidate the chemical sites of doped-phosphorus atoms in diamond. P 2p core level study shows two bulk components, providing spectroscopic evidence for multiple chemical sites of doped-phosphorus atoms. This indicates that only a part of doped-phosphorus atoms contribute to the formation of carriers. From a comparison with band calculations, possible origins for the chemical sites are discussed. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3554699]

DOI： 10.1063/1.3554699

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

We have studied electronic structures of U(Ru1-xRhx)(2)Si-2 employing ultrahigh-resolution laser angle-resolved photoemission spectroscopy to understand the effect of Rh substitution. A hole-like dispersive feature, which presumably has Rh d-band character, was observed in U(Ru1-xRhx)(2)Si-2 for both x = 0 and x = 0.03. However, although a heavy quasiparticle band appears in the hidden-order state of URu2Si2 (x = 0), it was not observed for x = 0.03 over the temperature range studied. In addition, it was found that energy-distribution curves at the Fermi vector of the hole-like band for x = 0.03 behave similarly to the Fermi-Dirac function. We also present the mapping of Fermi surfaces formed by the hole-like band, which stay nearly unaffected by Rh substitution.

DOI： 10.1088/1742-6596/273/1/012021

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：WILEY-V C H VERLAG GMBH  

Polycrystalline sample of hollandite-type titanium oxide KxTi8O16 has been prepared and studied by magnetization and X-ray photoelectron spectroscopy (XPS). Room temperature ferromagnetic behaviour is observed in the magnetic hysteresis measurements. XPS measurements of the Ti 2p(3/2) core-level finds that the titanium ions take a mixed valence of Ti4+ and Ti3+. In addition, the valence band spectrum reveals that the 3d electrons tend to localize on Ti3+ ions in the hollandite-type TiO2 lattice. Also, analysis of the valence band spectrum shows that the prepared sample is a wide-gap oxide with a band gap of 3.6 eV. These results indicate that the present hollandite-type KxTi8O16 sample can be classified as a TiO2-based wide-gap semiconductor with Curie temperature above room temperature. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

DOI： 10.1002/pssc.201000383

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In this work, we have performed resonant photoemission studies of Ca-intercalated graphite superconductor CaC6. Using photon energy of the Ca 2p-3d threshold, the photoemission intensity of the peak at Fermi energy (E-F) is resonantly enhanced. This result provides spectroscopic evidence for the existence of Ca 3d states at E-F, and strongly supports that Ca 3d state plays a crucial role for the superconductivity of this material with relatively high T-c. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2009.10.039

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The boron concentration dependence of the Si electronic structure of Si(1 00)2 x 1 surfaces were investigated by angle-resolved photoemission spectroscopy (ARPES). The ARPES spectra exhibit rigid shifts toward lower binding energy as the boron concentration increases. The band dispersion was obtained from fitting procedure, and it is found that the top of the valence band does not exceed the Fermi level even with a boron concentration 35 times larger than the critical concentration of the metal-insulator transition. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2009.11.028

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In this paper, we present the result of soft X-ray photoemission spectroscopy and its comparison with the density functional calculation. Although local density approximation seems to be a good starting point for describing the electronic structure of FeSe, the simulated spectrum poorly reproduced the structure around E-B = 2 eV. This result suggests the necessity of theoretical treatment beyond local density approximation. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2009.11.172

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We have performed soft x-ray photoemission spectroscopy (SXPES) and resonant photoemission spectroscopy (RPES) of the filled skutterudite superconductor PrPt4Ge12 in order to study the electronic structure of valence band and the character of Pr 4f. SXPES of PrPt4Ge12 measured with 1200 eV photon energy, where spectral contribution of Pr 4f is negligible, was found nearly identical with that of LaPt4Ge12, indicating similarity of Pt-Ge derived electronic states of the two compounds. Good correspondence with band calculations allows us to ascribe the dominant Ge 4p character of the density of states at the Fermi level (E-F). Pr 3d -&gt; 4f RPES shows that, although Pr 4f electrons in PrPt4Ge12 are not as strongly hybridized with conduction electrons near E-F as in PrFe4P12, there are finite Pr 4f contribution to the states near E-F in PrPt4Ge12. These PES results give the information of fundamental electronic structure for understanding the physical properties of the novel filled skutterudite superconductor PrPt4Ge12

DOI： 10.1143/JPSJ.79.124701

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The detail of electronic structures near the Fermi level in URu2Si2 has been investigated employing state-of-art laser angle-resolved photoemission spectroscopy. The observation of a narrow dispersive band near the Fermi level in the ordered state as well as its absence in a Rh-substituted sample strongly suggest that the emergence of the narrow band is a clear signature of the hidden-order transition. The temperature dependence of the narrow band, which appears at the onset of the hidden-order transition, invokes the occurrence of periodicity modification in the ordered state, which is shown for the first time by any spectroscopic probe. We compare our data to other previous studies and discuss possible implications.

DOI： 10.1103/PhysRevB.82.205108

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We use photoemission spectroscopy to study electronic structures of pristine and K-doped solid picene. The valence band spectrum of pristine picene consists of three main features with no state at the Fermi level (E(F)) while that of K-doped picene has three structures similar to those of pristine picene with new states near E(F), consistent with the semiconductor-metal transition. The K-induced change cannot be explained with a simple rigid-band model of pristine picene but can be interpreted by molecular-orbital calculations considering electron-intramolecular-vibration interaction. Excellent agreement of the K-doped spectrum with the calculations points to importance of electron-intramolecular-vibration interaction in K-doped picene.

DOI： 10.1103/PhysRevB.82.195114

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We present the results of angle-resolved photoemission spectroscopy (ARPES) measurements of metallic VO2 thin films. The VO2 thin films have been grown on TiO2 (0 0 1) single crystal substrates using pulsed laser deposition. The films exhibit a first-order metal-insulator transition (MIT) at 305K. In the ARPES spectra of the metallic phase for the films, the O 2p band shows highly dispersive feature in the binding energy range of 3-8 eV along the Gamma-Z direction. The periodicity of the dispersive band is found to be 2.2 A(-1) which is almost identical with the periodicity expected from the c-axis length of the VO2 thin films. The overall feature of the experimental band structure is similar to the band structure calculations. supporting that we have succeeded in observing the dispersive band of the O 2p state in the metallic VO2 thin film. The present work indicates that the ARPES measurements using epitaxial thin films are promising for determining the band structure of VO2. (C) 2010 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.elspec.2010.02.003

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We have prepared a CrO(2) thin film by chemical vapor deposition from a Cr(8)O(21) precursor and studied the bulk and surface physical properties. The CrO(2) thin film is grown on a TiO(2) (100) substrate by heating of a Cr(8)O(21) precursor and TiO(2) (100) substrate together in a sealed quartz tube. The prepared film is found from x-ray diffraction analysis to be an (100)-oriented single phase. The magnetization and resistivity measurements indicate that the film is a ferromagnetic metal with a Curie temperature of about 400 K. Cr 3s core-level and valence band photoelectron spectroscopy spectra reveal the presence of a metallic CrO(2) in the surface region of the film. Our work indicates that preparation from a Cr(8)O(21) precursor is promising for obtaining a CrO(2) thin film with the metallic surface. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3471811]

DOI： 10.1063/1.3471811

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-V C H VERLAG GMBH  

How annealing influences the morphology of a highly regioregular poly(3-hexylthiophene) (RR-P3HT) film at the substrate interface as well as the lateral inhomogeneity in the electronic structure of the film are elucidated. Whereas previous studies have reported that high-molecular-weight (MW) RR-P3HT films tend to show low crystallinity even, after annealing, it is found that high-MW RR-P3HT does show high crystallinity after annealing at high temperature for a long time. Photoemission electron microscopy (PEEM), X-ray photoemission spectroscopy, and ultraviolet photoemission spectroscopy results clearly resolve a considerable lateral inhomogeneity in the morphology of RR-P3HT film, which results in a variation of the electronic structure depending on the local crystallinity. The PEEM results show how annealing facilitates crystal growth in a high-MW RR-P3HT film.

DOI： 10.1002/adfm.201000116

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We have performed ultrahigh-resolution angle-resolved photoemission spectroscopy to directly study the large superconducting (SC) gap anisotropy of YNi2B2C. We succeed in measuring momentum (k) dependence of SC gap for individual Fermi surface (FS) sheets, which demonstrates complexity of SC gap in a phonon-mediated superconductor. Within measured k regions on FS sheets, we find a pointlike minimum of SC gap, whose k positions can be connected by the known nesting vector. This shows close correlation between the nesting vector and node formation.

DOI： 10.1103/PhysRevB.81.180509

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A hollandite-type K(x)Ti(8)O(16) polycrystalline sample has been prepared and studied by magnetization, resistivity and x-ray photoelectron spectroscopy (XPS). Room temperature ferromagnetic behavior is observed in the magnetic hysteresis measurement. The sample shows a semiconductive temperature dependence in the resistivity measurement. Analysis of the Ti 2p(3/2) core-level XPS spectrum indicates that the titanium ions have a mixed valence of Ti(4+) and Ti(3+). In addition, the valence band spectrum reveals that the 3d electrons tend to localize on Ti(3+) ions in the hollandite-type TiO(2) lattice. Also, analysis of the valence band spectrum shows that the prepared sample is a wide-gap oxide with a band gap of 3.6 eV. These results indicate that the present hollandite-type K(x)Ti(8)O(16) sample can be classified as a TiO(2)-based wide-gap semiconductor with Curie temperature above room temperature. Room temperature ferromagnetism (RTFM) decreases in the sample prepared under a strong reducing gas atmosphere, accompanied with the decrease in the resistivity. The results imply that the localized 3d electrons are responsible for the RTFM of the K(x)Ti(8)O(16) sample. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3369500]

DOI： 10.1063/1.3369500

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CeOs4Sb12 and CeFe4P12 are classified as Kondo semiconductors, which show coupled changes in electrical transport, thermodynamic and magnetic properties with a low-temperature semiconductor-like electrical resistivity. We have carried out core level and valence band photoemission spectroscopy on single crystal CeOs4Sb12 and CeFe4P12 to study their electronic structure and the evolution of states at the Fermi level as a function of temperature (similar to 10-300 K). The Ce 3d core level spectra show the presence of f(0), f(1) and f(2) final states with very different relative intensities in the two compounds. Single-impurity Anderson model calculations provide f electron counts of n(f) = 0.97 and 0.86 per Ce atom, suggestive of a low-and high-T-K (=single ion Kondo temperature) for CeOs4Sb12 and CeFe4P12, respectively. The high-resolution temperature-dependent near-Fermi level spectra show pseudogaps of energy similar to 50 meV and similar to 110 meV in the valence band density of states (DOS) of CeOs4Sb12 and CeFe4P12, respectively. The temperature dependence of the DOS at the Fermi level follows the change in effective magnetic moment estimated from magnetic susceptibility for both materials, confirming the Kondo nature of the pseudogap in CeOs4Sb12 and CeFe4P12. A compilation of measured pseudogaps using photoemission and optical spectroscopy identifies the charge gaps Delta(C) for Ce-based Kondo semiconductors and provides a direct relation with T-K given by Delta(C) similar to 2k(B)T(K). In conjunction with the known behaviour of the spin gaps Delta(S) similar to k(B)T(K), the results establish the coupled energy scaling of the spin and charge gaps in Kondo semiconductors.

DOI： 10.1088/0953-8984/22/9/095502

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Low voltage operation of picene thin film field-effect transistor (FET) has been realized with 40 nm thick SiO(2) gate dielectrics coated by two polymers, Cytop (TM) and polystyrene. The picene FETs operated in low absolute gate voltage vertical bar V(G)vertical bar below 15 V for Cytop (TM) coated SiO(2) and 30 V for polystyrene coated SiO(2) gate dielectrics, and they showed a significant O(2) gas sensing effect down to similar to 10 ppm. Photoemission spectrum clarified that O(2) molecules penetrate into the thin films at O(2)/picene mole ratio of 1: 1. X-ray diffraction pattern of picene thin films showed highly oriented growth on the polymer-coated SiO(2).

DOI： 10.1063/1.3257373

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Silicon carbide (SiC) is a candidate material for electronic devices to operate upon crucial environment. Electronic states of silicides and/or carbide/graphite formed in metal/SiC contact system is fundamentally important from the view point of device performance.
We study interface electronic structure of vanadium (V) thin-film deposited on 6H-SiC(0 0 0 1) Si-face by using a soft X-ray emission spectroscopy (SXES). For specimens of V(38 nm)/6H-SiC (substrate) contact systems annealed at 850 degrees C, the Si L(2,3) emission spectra indicate different shapes and peak energies from the substrate. The product of materials such as silicides and/or ternary materials is suggested. Similarly, the C K alpha emission spectra show the shape and peak energy characteristic of vanadium carbide including substrate 6H-SiC signal. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.apsusc.2009.05.120

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We have performed angle-resolved photoemission spectroscopy (ARPES) measurements of VO2 using epitaxial thin films and observed the band dispersion near the Fermi level (E-F) for this compound. The VO2 thin films have been grown on TiO2 (001) single-crystal substrates using pulsed laser deposition. The films exhibit a first-order metal-insulator transition (MIT) at 305 K. In the ARPES spectra of the metallic phase for the films, the O 2p band shows highly dispersive features in the binding-energy range of 3-8 eV along the Gamma-Z direction. Also, the V 3d state shows two dispersive bands around the Gamma point near E-F, indicative of two electron pockets centered at the Gamma point. Both electron pockets have an occupied bandwidth of about 0.4 eV. Assuming the parabolic energy bands around the Gamma point, the effective-mass ratios of the two electron pockets are estimated to be about 0.2 and 1. The present work indicates that the ARPES measurements using epitaxial thin films are promising for determining the band structure of VO2 and thus would play a crucial role to elucidate the mechanism of the MIT in VO2.

DOI： 10.1103/PhysRevB.80.125406

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We have measured Ca-intercalated graphite superconductor CaC6 (T-c = 11.2 K) by soft X-ray photoemission spectroscopy in order to understand the electronic structure. For the valence band, we observed several structures that correspond to those of calculated density of states with the partial density of states of Ca 3d at the Fermi level (E-F). We also observed core level spectra that are a very large asymmetric Ca 2p and asymmetric C 1s for CaC6, suggesting the existence of conduction electrons derived from Ca 3d and a charge transfer from Ca to graphene layer. These results provide spectroscopic evidence for PDOS of Ca 3d at E-F. From a comparison of electronic structure of CaC6 and other graphite intercalation compounds (GICs), we found the difference between CaC6 and other superconducting GICs, which provides deeper understanding of the superconductivity of CaC6. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2009.05.172

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We present the data from ultrahigh-resolution laser-excited photoemission spectroscopy on a newly discovered noncentrosymmetric superconcluctor Mg(10)Ir(19)B(16). We observed the evidences for the opening of superconducting gap where numerical fittings show that isotropic s-wave model can satisfactorily explain the behavior of spectra near the Fermi level. We also present the valence band photoemission spectrum and B K fluorescence spectrum to discuss the basic electronic structure of Mg(10)Ir(19)B(16). (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2009.05.175

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We have performed temperature (T)-dependent laser-photoemission spectroscopy of antiferromagnetic (AF) superconductor ErNi2B2C to study the electronic structure, especially the effect of AF ordering in T-dependent superconducting (SC) gap. To estimate the values of T-dependent SC gap, we fitted the experimental data by Dynes function having an anisotropic s-wave SC gap. From the fitting results we, find a sudden deviation from the BCS prediction just below T-N. This observation can be well explained by the theoretical model, indicating that the origin of anomalous T-dependence is competition between rapid evolution of AF molecular field and SC condensation energy. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2009.05.101

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DOI： 10.1103/PhysRevB.80.035420

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We have performed soft x-ray photoemission studies of Ca-intercalated graphite superconductor CaC6 (T-c=11.2 K). The valence-band spectrum shows six main structures, with a peak at the Fermi level (E-F), which correspond to those of calculated DOS. The intensity of the E-F peak is resonantly enhanced at the Ca 2p-3d threshold, providing spectroscopic evidence for the existence of Ca 3d electrons at E-F, which is confirmed experimentally for the first time. The Ca 2p core-level spectrum has a very large asymmetric line shape, which is further possible suggestion of the existence of Ca 3d derived conduction electrons at Ca sites. These results strongly support the picture where Ca 3d derived interlayer band plays a crucial role for the superconductivity of this material with relatively high T-c.

DOI： 10.1103/PhysRevB.80.035420

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A method to position samples with small cleaved regions has been developed to be applied to the angle-resolved photoemission spectroscopy (ARPES) which uses soft-x-ray synchrotron radiation focused down to 160x180 mu m(2). A long-working-distance optical microscope is used for the sample observation. A selected region on a sample can be optimally set at the position of measurements, which is realized by the spatial resolution of the photoelectron analyzer. Using this method, electronic band dispersions of bulk silicon have been measured by ARPES for a partially cleaved region with a size of similar to 200x500 mu m(2).

DOI： 10.1063/1.3124145

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A Reply to the Comment by J. D. Lee et al..

DOI： 10.1103/PhysRevLett.102.199702

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We have performed high-resolution soft x-ray B 1s core-level photoemission spectroscopy of heavily boron-doped diamond films in order to study the chemical sites of doped-boron atoms and their relation to the superconducting transition temperature (T-c). We find that B 1s core-level spectra exhibit several fine structures and three bulk components can he resolved from photon energy dependent studies and spectral fittings. These results indicate existence of several chemical environments of doped-boron atoms. One of the bulk components having the lowest binding energy can be assigned to a signal from substitutionally doped-boron atoms. Comparisons of bulk components with T-c suggest spectroscopic evidence for the importance of substitutionally doped-boron atoms for effective carrier and superconductivity.

DOI： 10.1143/JPSJ.78.034703

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We have pet-formed soft x-ray and Ultralligh-resolution laser-excited photoemission measurements on tetragonal FeSe, which was recently identified as a superconductor. Energy dependent study of valence band is compared to band Structure Calculations and yields a reasonable assignment of partial densities of states. However, the sharp peak near the Fermi level slightly deviates from the calculated energy position, giving rise to the necessity of self energy correction. We have also performed ultrahigh-resolution laser photoemission experiment on FeSe and observed the suppression of intensity around the Fermi level upon cooling.

DOI： 10.1143/JPSJ.78.034708

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We report the results of first photoemission spectroscopy on a newly discovered noncentrosymmetric superconductor Mg10Ir19B16. We measured the valence band structure of Mg10Ir19B16 using soft X-ray photoemission technique and found that the first principal calculation explains our observation well. In the bulk-sensitive laser-excited photoemission study, we observed that the intensity at the Fermi level decreases as we lower the sample temperature, indicative of the opening of superconducting energy gap. Using Dynes' function, we performed fitting analysis on the spectra of superconducting state.

DOI： 10.1143/JPSJ.78.034705

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Noncentrosymmetric superconductors Li2Pd3B and Li2Pt3B are studied by high-resolution photoemission spectroscopy. The valence band spectra of Li2Pd3B and Li2Pt3B are well explained by first principles band calculations. The sub-meV photoemission result shows that the size of the superconducting gap of Li2Pd3B is about 1.1 meV at 5.4 K with a reduced gap value (2 Delta(0)/k(B)T(c)) of about 4.5, which is greater than the Bardeen-Cooper-Schrieffer (BCS) value of 3.52. The obtained superconducting gap can be explained more clearly with an s-wave superconducting gap than with p- and d-wave superconducting gaps. This result indicates that Li2Pd3B is a conventional strong-coupling superconductor.

DOI： 10.1143/JPSJ.78.034711

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The temperature (T) dependent gap formation in the density of states (DOS) of FeSi has been investigated by angle-integrated laser photoemission spectroscopy (PES). With decreasing T, the evolution of a small gap (_60meV) at the Fermi level is observed in the DOS, indicating a p-type semiconducting character of this compound. The Fermi edge, which has been controversial in all past PES studies, is extremely small at 5K in accordance with transport and optical experiments. The T dependence of the gap, which gets smeared out quickly at high T as in optical conductivity spectrum, suggests the existence of a strong scattering mechanism beyond thermal excitations.

DOI： 10.1103/PhysRevB.72.233202

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We have performed soft X-ray angle-resolved photoemission spectroscopy (SXARPES) of a heavily boron-doped superconducting diamond film (T-c = 7.2 K) in order to Study the electronic structure near the Fermi level (E-F). Careful determination of measured momentum space that across F point in the Brillouin zone (BZ) and increase of an energy resolution provide further spectroscopic evidence that EF is located at the highly dispersive diamond-like bands, indicating that holes at the top of the diamond-like valence band play an essential role for the conducting properties of the heavily boron-doped superconducting diamond for this boron-doping region (effective carrier concentration of 1.6%). The SXARPES intensities at E-F were also mapped out over BZ to obtain experimental Fermi surface sheets and compared with calculations. Crown Copyright (c) 2008 Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jpcs.2008.06.006

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We have performed x-ray photoemission spectroscopy on the system of noncentrosymmetric Superconductor, Li-2(PdxPt1-x)(3)B. For Li2Pt3B, we found two major peaks with two other weak components, and the band calculations were in agreement with the observation. The assignment of valence band features using the calculated partial density of states determined that Pt 5d and B 2p contribute to the density of states at the Fermi level. The effect of antisymmetric spin-orbit coupling on the band Structure might have been probed. and the analysis on the effect of Pt incorporation into the system indicates the smooth evolution of electronic structures. We presented the measurements of core levels (Pd 3d, Pt 4f, and B 1s) and discussed the chemical bonding states and electronic structures from them.

DOI： 10.1143/JPSJ.77.104701

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Laser-excited photoemission spectroscopy is used to show that the doped carriers in metallic or superconducting diamond couple strongly to the lattice via high-energy (similar to 150 meV) optical phonons, with direct observations of localized Franck-Condon multiphonon sidebands appearing as Fermi-edge replicas. It exhibits a temperature-dependent spectral weight transfer from higher to lower energy sidebands and zero-phonon Fermi-edge states. The quantified coupling strength shows a systematic increase on lowering temperature, implicating its relation to the normal state transport and superconductivity.

DOI： 10.1103/PhysRevLett.100.166402

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We have performed temperature- (T-)dependent laser-photoemission spectroscopy of the antiferromagnetic (AF) superconductor ErNi(2)B(2)C to study the electronic-structure evolution reflecting the interplay between antiferromagnetism and superconductivity. The spectra at the superconducting (SC) phase show a very broad spectral shape. A T-dependent SC gap shows a sudden deviation from the BCS prediction just below T(N). This observation can be explained well by the theoretical model and thus represents the characteristic bulk electronic structure of the AF SC phase for the first time.

DOI： 10.1103/PhysRevLett.100.017003

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DOI： 10.1103/PhysRevB.76.169901

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Other Link： http://orcid.org/0000-0002-1124-0451

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We have used photoelectron emission microscopy (PEEM) and X-ray magnetic circular dichroism (XMCD) to study the effect of thin film thickness on the magnetic domain formation in La0.6Sr0.4MnO3 samples that were epitaxially grown on stepped SrTiO3 (00 1) substrates. The magnetic image exhibited a stripe structure elongated along the step direction, irrespective of film thickness, suggesting that uniaxial magnetic anisotropy induced by step-and-terrace structures plays an important role in the magnetic domain formation. Additional domains evolved gradually with increasing film thickness. In these domains, the direction of magnetization differed from the step direction due to biaxial magneto-crystalline anisotropy. The evolution of additional magnetic domains with increasing film thickness implies that a competition exists between the two anisotropies in LSMO films. (c) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.susc.2007.05.036

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Silicon carbide (SiC) is a candidate material for electronic devices to operate upon crucial environment. Electronic states of silicides and/or carbide/graphite formed in metal/SiC contact system are fundamentally important from the viewpoint of device performance.
We study interface electronic structure of iron thin film deposited on silicon (Si)- and carbon (C)-face of 4H-SiC(0 0 0 1) by using a soft X-ray emission spectroscopy (SXES). For specimens of Fe (50 nm)/4H-SiC (substrate) contact systems annealed at 700 and 900 degrees C, the Si L-2,L-3 emission spectra indicate different shapes and peak energies from the substrate depending on thermal-treated temperature. The product of materials such as silicides is suggested. Further, from comparison of Si L-2,L-3 emission spectra between Si- and C-face for the same annealing temperature at 700 degrees C, it is concluded that the similar silicides and/or ternary materials are formed on the two surfaces. However for those of 900 degrees C, the film on substrate is composed of the different silicide and/or ternary materials. (C) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.apsusc.2007.07.025

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

Charge ordering and superconductivity are observed in the phase diagrams of a variety of materials such as NbSe3, layered transition-metal dichalcogenides and high-temperature copper oxide superconductors, low-dimensional organics, Ba1-xKxBiO3 and so forth. Although both conventional charge-density-wave (CDW) and superconducting transitions show an energy gap in the single-particle density of states at the Fermi level ( E F), their physical properties are poles apart: insulating behaviour for the CDW and zero resistivity in superconductors. Consequently, these two ground states are believed to compete with each other. Here we provide evidence for maximized superconductivity at points in momentum (k) space that are directly connected by the CDW ordering vector. Temperature-dependent angle-resolved photoemission spectroscopy of 2H-NbSe2 across the CDW and superconducting transitions ( T-CDW similar to 33Kand T-c= 7.2 K, respectively) shows CDW-induced spectral-weight depletion at the same Fermi-surface-crossing k points, which evolve into the largest superconducting gaps. These k points also show the highest electron-phonon coupling and lowest Fermi velocities. Our results demonstrate that charge order can boost superconductivity in an electron-phonon coupled system, in direct contrast to the prevailing view that it only competes with superconductivity.

DOI： 10.1038/nphys699

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

By using photoelectron emission microscopy, we systematically investigated the magnetic domain in the La0.6Sr0.4MnO3 patterned thin film with several microns, in which we can control the ratio of a shape anisotropy to a step-induced uniaxial anisotropy in magnitude. We demonstrate that a cooperative effect of a step-induced uniaxial anisotropy with a shape anisotropy causes the formation of a single domain, whereas the competitive effect makes a multimagnetic domain. (C) 2007 American Institute of Physics.

DOI： 10.1063/1.2801699

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Language：English   Publisher：AMER PHYSICAL SOC  

DOI： 10.1103/PhysRevB.76.169901

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We have performed a sub-meV photoemission study on Mg(B1-xCx)(2) for x = 0-0.075. The superconducting (SC) transition temperature (T-c) changes systematically from 38 K to 24 K with increasing the carbon concentration. Because of the extremely high-energy resolution of sub-meV, two features corresponding to the two SC gaps originated from pi and sigma bands are clearly observed even in raw spectra up to x = 0.075. From numerical analyses, we found that the gap size of the sigma band is proportional to T-c, while that of the pi band is almost constant. The temperature and carbon concentration dependences of the two gaps are explained within a simple two-band model based on a mean field theory. Using the theoretical model, we deduced interband and intraband coupling strength constants. The transition temperature is dominated by the intraband coupling of the sigma band, while the interband coupling slightly increases with increasing the carbon concentration. The two-band model shows that the interband coupling slightly pushes up T-c. (c) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2007.03.071

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We have investigated the electronic structures of antiferromagnetic superconductor ErNi2B2C by using laser-photoemission spectroscopy. The observed spectra below the superconducting transition temperature (T-c = 9.0 K) show a very broad coherent peak and finite density of states at the Fermi level due to strong magnetic pair breaking effects. In addition, we find a dip structure in the temperature dependence of the superconducting order parameter just below the antiferromagnetic ordering temperature (T-N = 6.0 K). This behavior can be well interpreted as a result of competition between the rapid evolution of the antiferromagnetic molecular field and the increase of the superconducting condensation energy with decreasing temperature. (c) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2007.04.132

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The electronic structure near the Fermi level (E-F) of the beta-pyrochlore superconductor KOs2O6 is studied using laser-excited ultrahigh-resolution photoemission spectroscopy. The superconducting gap clearly opens across the superconducting transition (T-c=9.6 K), with the strong electron-phonon coupling value of 2 Delta(0)/k(B)T(c)&gt;= 4.56. A fitting analysis identifies clear anomalies at T-p=7.5 K in the temperature dependencies of the superconducting gap size and the quasiparticle relaxation lifetime. These anomalies and the fine spectral structures arising from phonons suggest that the existence of the rattling behavior of K ions significantly affects the superconductivity in KOs2O6.

DOI： 10.1103/PhysRevLett.99.117003

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Language：English   Publisher：ELSEVIER SCIENCE BV  

MgB2 shows the highest superconducting transition temperature among intermetallic compounds. Angle-resolved photoemission spectroscopy revealed their electronic structure, and the result is well explained within a simple band picture except for one band. Other photoemission studies on related materials of AlB2 and CaAlSi also show an extra band similar to that in MgB2. The direct observation of the superconducting gap confirmed a two-gap superconductivity. The larger and smaller gaps open at the sigma and pi bands, respectively. The carbon substitution effect on the two gaps was evaluated by using sub-meV photoemission spectroscopy. (c) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2006.12.003

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMERICAN PHYSICAL SOC  

Hard x-ray photoemission spectroscopy has been used to study intrinsic core-level electronic structure evolution of heavily boron-doped superconducting diamond films made with a microwave plasma-assisted chemical-vapor deposition method. The boron concentration dependent C 1s core-level spectra show systematic changes in the shift of the main peak and in the evolution of an additional feature at 1.1-1.3 eV lower binding energy than the main peak. In comparison to a low boron concentration nonsuperconducting diamond, the higher boron concentration doped diamond films show formation of several additional features in the B 1s core levels. Based on the present results, the local chemical environments around the doped boron atoms, the efficiency of hole doping by boron doping, and the implications for a recent x-ray absorption study are discussed.

DOI： 10.1103/PhysRevB.75.205117

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMERICAN PHYSICAL SOC  

We investigate the temperature (T)-dependent low-energy electronic structure of a boron-doped diamond thin film using ultrahigh resolution laser-excited photoemission spectroscopy. We observe a clear shift of the leading edge below T=11 K, indicative of a superconducting gap opening (Delta similar to 0.78 meV at T=4.5 K). The gap feature is significantly broad and a well-defined quasiparticle peak is lacking even at the lowest temperature of measurement (=4.5 K). We discuss our results in terms of disorder effects on the normal state transport and superconductivity in this system.

DOI： 10.1103/PhysRevLett.98.047003

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：MYU K K  

The valence band electronic structures of heavily boron-doped superconducting diamond films made by microwave plasma-assisted chemical vapor deposition (MPCVD) were investigated by hard X-ray photoemission spectroscopy (HXPES) and soft X-ray angle-resolved photoemission spectroscopy (SXARPES). The HXPES core-level spectrum of heavily boron-doped diamond shows a new feature at the lower binding energy side of the C Is main peak. The HXPES valence band spectrum of a heavily boron-doped superconducting diamond film shows a broader spectral shape than that of a lightly doped non-superconducting sample. The SXARPES results of homoepitaxial CVD films show clear valence band dispersions with a bandwidth of similar to 23 eV and the top of the valence band at the Gamma point in the Brillouin zone, which are well explained by the calculated valence band dispersions of pure diamond. Boron con centrati on-dependent band dispersions near the Fermi level (E-F) by SXARPES exhibit a systematic shift in E-F, indicating electron depopulation due to hole doping, and an increase in the line shape corresponding to the broader density of states observed by HXPES. These results indicate that holes in the top of the valence band are responsible for the metallic states leading to superconductivity at low temperatures. The HXPES C Is core-level spectra of lightly boron-doped non-superconducting and heavily boron-doped superconducting films are also shown.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：MATERIALS RESEARCH SOC  

Recent photoemission studies on heavily boron-doped superconducting diamond films, reporting the electronic structure evolution as a function of boron concentrations, are reviewed. From soft X-ray angle-resolved photoemission spectroscopy, which directly measures electronic band dispersions, depopulation of electrons (or formation of hole pockets) at the top of the valence band were clearly observed. This indicates that the holes at the top of the valence bands are responsible for the metallic properties and hence superconductivity at lower temperatures. Hard X-ray photoemission spectroscopy observed shift of the main C Is core level and intensity evolution of a lower binding energy additional structure, suggesting chemical potential shift, carrier doping efficiency by boron doping, and possibility of boron-related cluster formations.

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Language：Japanese   Publisher：The Physical Society of Japan  

DOI： 10.11316/jpsgaiyo.62.2.3.0_665_1

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Language：Japanese   Publisher：The Physical Society of Japan  

DOI： 10.11316/jpsgaiyo.62.2.3.0_665_2

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMERICAN PHYSICAL SOC  

The cobalt oxide superconductor NaxCoO2 center dot yH(2)O is studied by angle-resolved photoemission spectroscopy. We report the Fermi surface (FS) topology and electronic structure near the Fermi level (E-F) in the normal state of NaxCoO2 center dot yH(2)O. Our result indicates the presence of the hexagonal FS centered at the Gamma point, while the small pocket FSs along Gamma-K direction are absent, similar to NaxCoO2. The top of the e(g)(') band, which is expected in band calculations to form the small pocket FSs, extends to within similar to 30 meV below E-F, closer to E-F than in NaxCoO2. We discuss its possible role in superconductivity, comparing with other experimental and theoretical results.

DOI： 10.1103/PhysRevLett.97.267003

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We present laser-excited ultrahigh-resolution photoemission spectroscopy of YNi2B2C and ErNi2B2C. The extremely high energy resolution enables us to discuss the superconducting (SC) gap function of YNi2B2C and to measure the density of states which is coexisted between superconductivity and antiferromagnetism in ErNiAC. For YNi2B2C, s + g-wave SC gap symmetry cannot reproduce the experimental data, implying importance of taking the complicated electronic structures into consideration. For ErNi2B2C, the observed coherent peak is very broad and finite density of state exists at Fermi level. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2006.03.074

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Magnetic domain formation in thin films of La1-xSrxMnO3 (LSMO) with x=0.2 and 0.4 epitaxially grown on stepped SrTiO3 (001) substrates has been investigated by photoelectron emission microscopy. The magnetic domains show a stripe structure elongated along the step directions, indicating uniaxial magnetic anisotropy induced by step structures. We have also found that the magnetization of the magnetic domains is slightly meandering at domain boundaries. The existence of the additional structures suggests that the magnetic domains in LSMO films are also influenced by biaxial magnetocrystalline anisotropy. The direct observation of the magnetic domain structures suggests that the competition between the two magnetic anisotropies may play an important role in magnetic properties in LSMO films. (c) 2006 American Institute of Physics.

DOI： 10.1063/1.2347895

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

We study the superconducting gap of the filled skutterudite superconductor LaRu4P12 (transition temperature T-c = 7.2 K) using a newly developed sub-meV resolution photoemission spectrometer with a vacuum-UV laser as a light source. The result at 4.8 K shows the opening of a superconducting gap, whose spectral shape is well explained with an isotropic s-wave Dynes function using a gap size of 1.3 meV. This corresponds to the reduced gap 2 Delta(0)/k(B)T(c) of 4.2. The temperature dependence of the superconducting gap shows fairly good agreement with the conventional BCS theory. The present results are consistent with those of a moderately strong-coupling and isotropic s-wave superconductor.

DOI： 10.1143/JPSJ.75.064711

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We have performed laser-excited ultrahigh-resolution photoemission spectroscopy of Y(Ni1-xPt2)(2)B2C (x = 0.0, 0.2) and numerical simulation using s, anisotropic s and even s + g symmetry to study the superconducting gap anisotropy in borocarbide supercodnuctors. We observed the reduction of gap anisotropy in the x = 0.2 sample in comparison to the x = 0.0 sample, confirming the anisotropic s-wave gap. We also found that the spectrum of x = 0.0 at 3.5 K cannot be fitted well with the s + g-wave symmetry, implying that the s + g symmetry is too simple to describe over all supercodncuting gap electronic Structure of borocarbides. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2006.01.204

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC APPLIED PHYSICS  

We have performed photoelectron emission microscopy measurements with hard X-rays (HX-PEEM) and have shown that HX-PEEM has a spatial resolution higher than 106 nm for observing the sample surface and can detect signals from a Au layer buried under a 50-nm-thick Co layer. These results mean that HX-PEEM has a probing depth one order of magnitude deeper than that of PEEM with soft X-rays. This deeper probing depth with HX-PEEM provides a new opportunity to investigate electronic and/or magnetic structures of buried layers under a thick overlayer. The notable advantages and potential applications of HX-PEEM are discussed.

DOI： 10.1143/JJAP.45.1886

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We study the electronic structure of Mott-Hubbard systems SrVO3 and CaVO3 with bulk and surface-sensitive high-resolution photoemission spectroscopy, using a vacuum ultraviolet laser, synchrotron radiation, and a discharge lamp (h nu=7-21 eV). A systematic suppression of the density of states (DOS) within similar to 0.2 eV of the Fermi level (E-F) is found on decreasing photon energy, i.e., on increasing bulk sensitivity. The coherent band in SrVO3 and CaVO3 is shown to consist of surface and bulk-derived features, separated in energy. The stronger distortion on surface of CaVO3 compared to SrVO3 leads to a higher surface metallicity in the coherent DOS at E-F, consistent with recent theory.

DOI： 10.1103/PhysRevLett.96.076402

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

We review our angle-resolved photoemission spectroscopy (ARPES) results across superconducting transitions in 2H-NbSe2 and MgB2 and also show recent results on YNi2B2C. For 2H-NbSe2 and MgB2, we observe that the superconducting gap exhibits clear Fermi surface (FS) sheet dependence for two types of FS sheets with different characters, thus providing evidence that the FS sheet dependence is a general feature for a superconductor having multiple FS sheets of different character. In YNi2B2C, we show experimental FSs, and momentum-dependent superconducting gap showing large anisotropy observed on a single FS. These ARPES results provide direct evidence for and importance of momentum-dependent superconducting gaps of phonon-mediated superconductors. (c) 2006 Published by Elsevier Ltd.

DOI： 10.1016/j.jpcs.2005.10.125

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

We have studied the electronic structure near the Fermi level of the layered cobaltate superconductor Na(0.35)CoO2 center dot 1.3H(2)0 using laser-excited ultrahigh-resolution photoemission spectroscopy (PES). In the normal state, we observed the formation of a pseudogap with an energy scale of similar to 20 meV. Across the superconducting transition, we observed the leading-edge shift at 3.5 K compared to that at 10 K, indicating superconducting gap (SC gap) opening having a gap value Delta of 0.5-0.7 meV. The energy scale of the SC gap is similar to 20 times smaller than that of the pseudogap, suggesting the existence of competing order parameters at low temperatures. (c) 2006 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jpcs.2005.10.124

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST PHYSICS  

We have performed high-resolution angle-resolved photoemission spectroscopy (ARPES) of the borocarbide superconductor YNi2B2C to Study the Fermi surface (FS) topology of the anisotropic s-wave superconductor. We successfully observed experimental valence band dispersions and Fermi surfaces (FSs), and indicated two nested parts connected by the known nesting vector Q similar to(0.55,0,0).

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

Valence band spectra of icosahedral quasicrystals and their 1/1 cubic approximants in the Cd-Yb and Cd-Ca systems are investigated systematically. A sharp Fermi edge, indicative of sizable density of states at the Fermi level (EF), is clearly observed for all the compounds and EF is located not at the bottom of the pseudogap but at its shoulder. In addition, a peak structure appears just below E-F for the Cd-Ca system, which is also found for the Cd-Yb system when subtracting a Yb 4f derived doublet. Ca 2p-3d resonant photoemission spectroscopies on the Cd-Ca compounds reveal the existence of Ca 3d states at EF, indicating that the 3d states do contribute to the peak structure at EF, which is attributed to hybridization with the Cd sp states. The low-temperature structural properties of the Cd-based compounds are also reported. An occurrence of a structural phase transition at 110K and 100K for Cd6Yb and Cd6Ca, respectively, is well documented. The transitions are understood as a consequence of an orientational ordering of Cd-4 tetrahedron located at the center of the icosahedral clusters. It is suggested that two orientations of the Cd4 tetrahedron alternatively occur along the [110] direction of the high temperature bcc phase.

DOI： 10.1080/14786430500267778

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATL INST MATERIALS SCIENCE  

We have investigated the low-energy electronic state of boron-doped diamond thin film by the laser-excited photoemission spectroscopy. A clear Fermi-edge is observed for samples doped above the semiconductor-metal boundary, together with the characteristic structures at 150 x n meV possibly due to the strong electron-lattice coupling effect. In addition, for the superconducting sample, we observed a shift of the leading edge below T-c indicative of a superconducting gap opening. We discuss the electron-lattice coupling and the superconductivity in doped diamond. (c) 2006 NIMS and Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.stam.2006.05.008

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATL INST MATERIALS SCIENCE  

We have performed soft X-ray angle-resolved photoemission spectroscopy (SXARPES) of microwave plasma-assisted chemical vapor deposition diamond films with different B concentrations in order to study the origin of the metallic behavior of superconducting diamond. SXARPES results clearly show valence band dispersions with a bandwidth of similar to 23 eV and with a top of the valence band at gamma point in the Brillouin zone, which are consistent with the calculated valence band dispersions of pure diamond. Boron concentration-dependent band dispersions near the Fermi level (E-F) exhibit a systematic shift of E-F, indicating depopulation of electrons due to hole doping. These SXARPES results indicate that diamond bands retain for heavy boron doping and holes in the diamond band are responsible for the metallic states leading to superconductivity at low temperature. A high-resolution photoemission spectroscopy spectrum near E-F of a heavily boron-doped diamond superconductor is also presented. (c) 2006 NIMS and Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.stam.2006.02.014

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The temperature (T) dependent gap formation in the density of states (DOS) of FeSi has been investigated by angle-integrated laser photoemission spectroscopy (PES). With decreasing T, the evolution of a small gap (similar to 60 meV) at the Fermi level is observed in the DOS, indicating a p-type semiconducting character of this compound. The Fermi edge, which has been controversial in all past PES studies, is extremely small at 5 K in accordance with transport and optical experiments. The T dependence of the gap, which gets smeared out quickly at high T as in optical conductivity spectrum, suggests the existence of a strong scattering mechanism beyond thermal excitations.

DOI： 10.1103/PhysRevB.72.233202

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The Luttinger surface of an organic metal (TTF-TCNQ), possessing charge order and spin-charge separated band dispersions, is investigated using temperature-dependent angle-resolved photoemission spectroscopy. The Luttinger surface topology, obtained from momentum distribution curves, changes from quasi-2D (dimensional) to quasi-1D with temperature. The high temperature quasi-2D surface exhibits 4k(F) charge-density-wave (CDW) superstructure in the TCNQ derived holon band, in the absence of 2k(F) order. Decreasing temperature results in quasi-1D nested 2k(F) CDW order in the TCNQ spinon band and in the TTF surface. The results establish the link in momentum space between charge order and spin-charge separation in a Luttinger liquid.

DOI： 10.1103/PhysRevLett.95.246402

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

The physical properties of lightly doped semiconductors are well described by electronic band- structure calculations and impurity energy levels(1). Such properties form the basis of present- day semiconductor technology. If the doping concentration n exceeds a critical value n(c), the system passes through an insulator- to- metal transition and exhibits metallic behaviour; this is widely accepted to occur as a consequence of the impurity levels merging to form energy bands(2). However, the electronic structure of semiconductors doped beyond nc have not been explored in detail. Therefore, the recent observation of superconductivity emerging near the insulator- to- metal transition(3) in heavily boron- doped diamond(4,5) has stimulated a discussion on the fundamental origin of the metallic states responsible for the superconductivity. Two approaches have been adopted for describing this metallic state: the introduction of charge carriers into either the impurity bands(6) or the intrinsic diamond bands(7-9). Here we show experimentally that the doping- dependent occupied electronic structures are consistent with the diamond bands, indicating that holes in the diamond bands play an essential part in determining the metallic nature of the heavily boron- doped diamond superconductor. This supports the diamond band approach and related predictions, including the possibility of achieving dopant- induced superconductivity in silicon and germanium(7). It should also provide a foundation for the possible development of diamond- based devices(10).

DOI： 10.1038/nature04278

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Hard x-ray photoemission spectroscopy (PES) of Cu core electronic states, with a probing depth of similar to 60 angstrom, is used to show that the Zhang-Rice singlet feature is present in La2CuO4 but is absent in Nd2CuO4. Hole and electron doping in La2-xSrxCuO4 (LSCO) and Nd2-xCexCuO4 (NCCO) result in new well-screened features which are missing in soft x-ray PES. Impurity Anderson model calculations establish screening from doped states as its origin, which is strongly suppressed within 15 angstrom of the surface. Complemented with x-ray absorption spectroscopy, the small chemical-potential shift in core levels (similar to 0.2 eV) are shown to be consistent with modifications of valence and conduction band states spanning the band gap (similar to 1 eV) upon hole and electron doping in LSCO and NCCO.

DOI： 10.1103/PhysRevLett.95.177002

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Language：English   Publishing type：Research paper (scientific journal)  

"Sub-meV" resolution photoemission spectroscopy was used to study carbon-substitution dependence of the multiple superconducting gap of Mg (B1-x Cx) 2. Two features corresponding to σ and π gaps are clearly observed in the raw spectra up to carbon concentration x=0.075. The observed x dependence of the two gaps shows a qualitatively different behavior: the σ gap is proportional to Tc while the π gap shows negligible change. Doping as well as temperature dependence can be explained within the two-band mean-field theory. Implications from the present study are discussed. © 2005 The American Physical Society.

DOI： 10.1103/PhysRevB.72.064527

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The temperature dependence of the low-energy electronic state for a typical quasi-one-dimensional material K0.3MoO3 has been investigated using high-resolution angle-resolved photoemission spectroscopy. We observed well-defined open quasi-one-dimensional Fermi surfaces (FSs) throughout the entire Brillouin zone. The warping of the FSs arising from the finite interchain transfer is also clearly discernible; this seems to be reduced at low temperature. It is indicative of the variation of the nesting condition, from imperfect to nearly perfect, on decreasing the temperature across the Peierls transition.

DOI： 10.1088/0953-8984/17/32/007

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

"Sub-meV" resolution photoemission spectroscopy was used to study carbon-substitution dependence of the multiple superconducting gap of Mg(B1-xCx)(2). Two features corresponding to sigma and pi gaps are clearly observed in the raw spectra up to carbon concentration x=0.075. The observed x dependence of the two gaps shows a qualitatively different behavior: the sigma gap is proportional to T-c while the pi gap shows negligible change. Doping as well as temperature dependence can be explained within the two-band mean-field theory. Implications from the present study are discussed.

DOI： 10.1103/PhysRevB.72.064527

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The energy-momentum (epsilon-k) dispersion and the shape of the Fermi surface in the (Bi,Pb)(2)(Sr,La)(2)CuO6+delta (Bi2201) superconductors with various hole concentrations were determined by the high resolution angle-resolved photoemission spectroscopy. On the basis of the epsilon-k dispersion thus obtained, temperature and hole-concentration dependence of the thermoelectric power S(T) was calculated within the framework of the Boltzmann transport theory. S(T) of the optimally and overdoped samples was quantitatively reproduced by the present calculation, strongly indicating that characteristics in S(T) of the high-T-c cuprates under the optimally to overdoped conditions is unambiguously caused by the band structure characterized by the van Hove singularity near the Fermi level.

DOI： 10.1103/PhysRevB.72.024533

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We study the bulk electronic structure of Na(x)CoO(2 center dot)yH(2)O using Hard X-ray (HX, hv = 5.95KeV) synchrotron photoelectron spectroscopy (PES). The Co 2p core level spectra show well-separated Co3+ and Co4+ ions. Cluster calculations suggest low spin Co3+ and Co4+ character, and a moderate on-site Coulomb correlation energy U-dd similar to 3-5.5 eV. Photon-dependent valence band PES identifies Co 3d and 0 2p derived states, in near agreement with band structure calculations. We discuss the importance of HX-PES for studying correlated transition metal oxides. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.nima.2005.05.021

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We have developed an ultrahigh-resolution photoemission system using a quasi continuous wave (quasi-CW) VUV laser. The photon energy of the VUV laser is 6.994 eV. It has a line width of 260 mu eV and a repetition rate of 80 MHz. The photon energy of this laser is the highest for a quasi-CW laser. Using a quasi-CW laser removes the space charge effect and hence does not degrade the energy resolution. For the electron analyser, we use the newly designed ultrahigh-resolution photoelectron analyser GAMMADATA-SCIENTA R4000. The analyser resolution is about 250 mu eV with a wide collecting angle of about 38 degrees. The total energy resolution of this system is measured to be 360 mu eV for the Fermi edge of gold. Using this system, we have measured several low transition temperature superconductors and succeeded to observe the bulk superconducting state of these materials. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.elspec.2005.01.154

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Relaxation time of Bloch states at the Fermi level (4), Fermi velocity (110, and mean free path (IF) of the conduction electrons in the (Bi, Pb)(2)(Sr, La)(2)CuO6+delta (Bi2201) superconductors with various hole-concentration (p) were determined by the angle-resolved photoemission spectroscopy (ARPES) measurement with high energy- and momentum-resolutions; Delta E = 10 meV and Delta k = 0.005 angstrom(-1). We found that tau(F) at k(F) along (0, 0)-(pi, pi) cut is kept constant over a wide Hole-concentration range while it in the vicinity of (pi, 0) is drastically shortened with decreasing p and eventually reaches nearly zero in the under-doped sample with the transition temperature of 27 K. By calculating electrical resistivity and comparing it with the measured one, we confirmed that tau(F), v(F), and l(F) determined by the ARPES measurement can be used for the quantitative estimation of the electrical transport properties, such as the electrical resistivity, in high-T-c cuprates. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.elspec.2005.01.198

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMERICAN PHYSICAL SOC  

We report bulk-sensitive hard x-ray (h nu=5.95 keV) core-level photoemission spectroscopy (PES) of single crystal V1.98Cr0.02O3 and the high-T-c cuprate Bi2Sr2CaCu2O8+delta (Bi2212). V1.98Cr0.02O3 exhibits low binding energy "satellites" to the V 2p "main lines" in the metallic phase, which are suppressed in the antiferromagnetic insulator phase. In contrast, the Cu 2p spectra of Bi2212 do not show temperature-dependent features, but a comparison with soft x-ray PES indicates a large increase in the 2p(5)3d(9) satellites or 3d(9) weight in the bulk. Cluster model calculations, including a full multiplet structure and a screening channel derived from the coherent band at the Fermi energy, give very satisfactory agreement with the experiments.

DOI： 10.1103/PhysRevB.71.155102

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMERICAN PHYSICAL SOC  

Angle-resolved photoemission spectroscopy of ZrTe3 has been performed from room temperature (T) down to 6 K (across T-CDW=63 K) to study the charge-density-wave (CDW) fluctuation effects in a metallic CDW compound having Fermi surface (FS) sheets with differing dimensionality. While spectra on the three-dimensional (3D) FS show typical Fermi-Dirac functionlike T dependence, those along the quasi-one-dimensional (1D) FS show formation of a pseudogap, starting at a much higher T than T-CDW. Simultaneously, a van-Hove singularity consisting of the quasi-1D FS intersecting the 3D FS shows an increase of coherence. This demonstrates the role of CDW fluctuations on the spectral function, and relation to the dimensionality of the states, in a metallic CDW system.

DOI： 10.1103/PhysRevB.71.140504

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We use x-ray photoemission spectroscopy to study the electronic structure of new superconductor Li 2Pd 3B. Binding energies of the Pd 3p and 3d indicate valency that is intermediate between Pd metal (0±) and PdO(2+), or effectively Pd 4d 9 configuration. Comparison of satellite features in core level spectra of Li 2Pd 3B with that of PdO suggests that a larger hybridization strength of Li 2Pd 3B than that of PdO. The valence band spectrum is in excellent agreement with first principle band structure calculations, allowing us to ascribe the states at the Fermi level responsible for the superconductivity to be mainly Pd 4d hybridized with B 2p and Li 2p. These observations indicate that correlation effects play a negligible role for the physical properties of the new superconductor. ©2005 The American Physical Society.

DOI： 10.1103/PhysRevB.71.092507

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We use x-ray photoemission spectroscopy to study the electronic structure of new superconductor Li2Pd3B. Binding energies of the Pd 3p and 3d indicate valency that is intermediate between Pd metal (0) and PdO(2+), or effectively Pd 4d(9) configuration. Comparison of satellite features in core level spectra of Li2Pd3B with that of PdO suggests that a larger hybridization strength of Li2Pd3B than that of PdO. The valence band spectrum is in excellent agreement with first principle band structure calculations, allowing us to ascribe the states at the Fermi ,level responsible for the superconductivity to be mainly Pd 4d hybridized with B 2p and Li 2p. These observations indicate that correlation effects play a negligible role for the physical properties of the new superconductor.

DOI： 10.1103/PhysRevB.71.092507

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We used low-temperature ultrahigh-resolution (360 mueV) photoemission spectroscopy with a laser as a photon source (Laser-PES) to study the superconducting (SC) gap of an f-electron superconductor CeRu2. The unique combination of the large escape depth expected from the known universal behavior and extremely high-energy resolution has enabled us to directly measure the bulk SC gap of an f-electron superconductor for the first time. The present study provides direct evidence for an anisotropic SC gap in CeRu2, and also demonstrates the potential of Laser-PES in investigating unconventional superconductivity realized in correlated d- and f-electron superconductors.

DOI： 10.1103/PhysRevLett.94.057001

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We have studied the temperature (T)-dependent electronic structure near the Fermi level of the layered cobaltate superconductor, Na0.35CoO2.1.3H(2)O, and related materials, using laser-excited ultrahigh-resolution photoemission spectroscopy. We observe the formation of a pseudogap (PG) with an energy scale of similar to20 meV in Na0.35CoO2.1.3H(2)O and Na0.35CoO2.0.7H(2)O, which is clearly absent in Na0.7CoO2. The energy scale of the PG is larger than the expected value for the superconducting gap, suggesting an additional competing order parameter at low T. We discuss implications of the PG in relation to available transport and magnetic susceptibility results.

DOI： 10.1103/PhysRevB.71.020505

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We have studied the temperature (T)-dependent electronic structure near the Fermi level of the layered cobaltate superconductor, Na 0.35CoO 2·1.3H 2O, and related materials, using laser-excited ultrahigh-resolution photoemission spectroscopy. We observe the formation of a pseudogap (PG) with an energy scale of ∼20 meV in Na 0.35CoO 2·1.3H 2O and Na 0.35CoO 2·0.7H 2O, which is clearly absent in Na 0.7CoO 2. The energy scale of the PG is larger than the expected value for the superconducting gap, suggesting an additional competing order parameter at low T. We discuss implications of the PG in relation to available transport and magnetic susceptibility results. ©2005 The American Physical Society.

DOI： 10.1103/PhysRevB.71.020505

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We study the electronic structure of EuNi2(Si 0.2Ge0.8)2, which exhibits a temperature dependent mixed valence transition, using 4d-4f resonant photoemission spectroscopy (RESPES), x-ray absorption spectroscopy (XAS) and temperature-dependent ultraviolet photoemission spectroscopy (UPS). The RESPES studies identify the divalent and trivalent Eu 4f character density of states (DOS) which participate in the valence transition. Using the photoionization cross section variation as a function of photon energy, we discuss the Eu, Ni, and Ge-Si partial DOS in the valence band. The bulk divalent Eu 4f character states are centered at a binding energy of about 0.75 eV, significantly away from the Fermi level. While the surface divalent feature is negligibly affected, the spectra obtained using He IIα UPS exhibit temperature dependent bulk Eu 4f character states. The bulk divalent spectral weight is transferred to the high energy trivalent states, across the valence transition temperature, T υ ∼ 80 K. The He Iα UPS also exhibit spectral intensity changes across Tυ. The non-f character conduction band states at and near the Fermi level exhibit spectral weight changes up to 350 meV with a small energy (∼25 meV) temperature dependent pseudogaplike feature. The results suggest an increase in effective hybridization strength between the conduction and 4f electrons in the low temperature nearly trivalent phase. While the 4f character changes across Tυ are qualitatively consistent with change in valence configurations, the temperature dependent spectral changes in the non-f character DOS indicate direct participation in the valence transition in EuNi2(Si 0.2Ge0.8)2.

DOI： 10.1103/PhysRevB.70.235105

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We study the electronic structure of EuNi2(Si0.2Ge0.8)(2), which exhibits a temperature dependent mixed valence transition, using 4d-4f resonant photoemission spectroscopy (RESPES), x-ray absorption spectroscopy (XAS) and temperature-dependent ultraviolet photoemission spectroscopy (UPS). The RESPES studies identify the divalent and trivalent Eu 4f character density of states (DOS) which participate in the valence transition. Using the photoionization cross section variation as a function of photon energy, we discuss the Eu, Ni, and Ge-Si partial DOS in the valence band. The bulk divalent Eu 4f character states are centered at a binding energy of about 0.75 eV, significantly away from the Fermi level. While the surface divalent feature is negligibly affected, the spectra obtained using He IIalpha UPS exhibit temperature dependent bulk Eu 4f character states. The bulk divalent spectral weight is transferred to the high energy trivalent states, across the valence transition temperature, T(v)similar to80 K. The He Ialpha UPS also exhibit spectral intensity changes across T-v. The non-f character conduction band states at and near the Fermi level exhibit spectral weight changes up to 350 meV with a small energy (similar to25 meV) temperature dependent pseudogaplike feature. The results suggest an increase in effective hybridization strength between the conduction and 4f electrons in the low temperature nearly trivalent phase. While the 4f character changes across T-v are qualitatively consistent with change in valence configurations, the temperature dependent spectral changes in the non-f character DOS indicate direct participation in the valence transition in EuNi2(Si0.2Ge0.8)(2).

DOI： 10.1103/PhysRevB.70.235105

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We have performed photoemission (PE) spectroscopy of beta-ZrNCl and NaxZrNCl to study intercalation-induced change in the electronic structure. The valence band PE spectrum of beta-ZrNCl shows a small intensity region within similar to2.5 eV of E-F with a very small hump at E-F that can be ascribed to hydrogen impurity states. These provide spectroscopic evidence that beta-ZrNCl is an n-type semiconductor. Upon Na intercalation, new states possibly involving hybridized Zr4d and N2p systematically evolve at E-F, with a negligible shift in the valence band structures. Similar results in beta-HfNCl and NaxHfNCl indicate that these results represent common electronic features of the layered nitride chloride superconductors.

DOI： 10.1103/PhysRevB.70.193103

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We have performed photoemission (PE) spectroscopy of β-ZrNCl and NaxZrNCl to study intercalation-induced change in the electronic structure. The valence band PE spectrum of β-ZrNCl shows a small intensity region within ∼2.5 eV of EF with a very small hump at E F that can be ascribed to hydrogen impurity states. These provide spectroscopic evidence that β-ZrNCl is an n-type semiconductor. Upon Na intercalation, new states possibly involving hybridized Zr4d and N2p systematically evolve at EF, with a negligible shift in the valence band structures. Similar results in β-HfNCl and NaxHfNCl indicate that these results represent common electronic features of the layered nitride chloride superconductors.

DOI： 10.1103/PhysRevB.70.193103

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Language：English   Publisher：AMERICAN PHYSICAL SOC  

DOI： 10.1103/PhysRevB.70.159902

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

The Fermi surface and superconducting gap of MgB2 have been studied by angle-resolved photoemission spectroscopy. Two Fermi surfaces composed of the a band and the pi band are clearly observed without contaminated by the "surface-derived" band. The high-resolution photoemission spectra show that superconducting gaps with values of 5.5 and 2.2 meV open on the sigma band and the pi band, respectively, both of which close at the bulk transition temperature (T-c). This result provides an experimental evidence for the two-band superconductivity on MgB2 and, moreover, validates the role of momentum-dependent electron-phonon coupling as the origin of multiple-gap superconductivity as well as the high transition temperature (T-c) of MgB2. (C) 2004 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2004.01.037

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Resonant photoemission spectroscopy, soft X-ray emission spectroscopy, soft X-ray absorption spectroscopy, and high-resolution (DeltaE approximate to 10 meV) ultraviolet photoemission spectroscopy were performed on a layered cobalt oxide, Ca3Co4O9, which has attracted interests as one of the potential thermoelectric materials because of its possession of high thermoelectric power S, relatively low electrical resistivity rho, and small thermal conductivity kappa. A narrow band of 1.5-2 eV in width was observed in the photoemission spectra with its center at 1.0 eV below the Fermi level (E-F). This peak turns out to be less significant when Co 2p-3d resonance takes place, indicating that it consists mainly of O 2p and of small amount of Co 3d component. Since E-F is located near the high-energy edge of this narrow band, the density of states at E-F is finite but negligibly small at room temperature. An energy-gap across E-F opens below 50 K with decreasing temperature. This development of the energy-gap causes the insulating behavior; divergence both in electrical resistivity and Hall coefficient. We calculated thermoelectric power S(T) using the photoemission spectra near E-F. The calculated S(T) shows almost half of the measured value. Large thermoelectric power up to 200 muV/K observed for the Ca3CO4O9 is closely related to the metallic electron transport in a less dispersive band with EF near its band-edge. (C) 2004 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.elspec.2004.02.013

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Energy-momentum (E-k) dispersion and the shape of the Fermi surface (FS) in the (Bi,Pb)(2)(Sr,La)(2)CuO6+delta (Bi2201) superconductors of various hole-concentrations were investigated by the angle-resolved photoemission spectroscopy (ARPES) with high energy and momentum resolutions (DeltaE = 10 meV and Deltak = 0.005 Angstrom(-1)). The E-k dispersion is kept rigid and the chemical potential moves towards lower or higher energies with increasing or decreasing hole-concentration, respectively, over a wide hole-concentration range from the nearly optimal-doped condition with a transition temperature of 32 K to a heavily over-doped condition where the superconducting transition disappears. By comparing the shape of the FS determined for the present Bi2201 with that reported for the Bi2Sr2CaCu2O8+delta (Bi2212), we found that the area surrounded by the FS in the Bi2201 is fairly larger than that in the Bi2212. Estimated hole-concentration in the Bi2201 was found to vary from 0.25 to 0.43 holes/Cu, that is more than twice as large as that in the optimally doped La(2-)xSrCuO(4) (LSCO) and Bi2212 superconductors of 0.17 holes/Cu. (C)2004 Elsevier B.V. All fights reserved.

DOI： 10.1016/j.elspec.2004.02.104

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：INST PURE APPLIED PHYSICS  

An aluminum film with a thickness of 1500Angstrom has been used as a filter for the He Ialpha resonance line (21.2182eV) from a microwave-driven high-flux discharging lamp to reduce the degradation of sample surfaces during photoemission spectroscopy (PES) measurements. A marked increase in the lifetime of sample surfaces, which overcomes a similar to90% intensity reduction, has been observed. The thin-film filter, if combined with a high-flux discharging lamp, provides clean vacuum ultraviolet lights for reliable PES measurements with an ultrahigh resolution.

DOI： 10.1143/JJAP.43.3618

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High-energy (hv = 5.95 keV) synchrotron photoemission spectroscopy (PES) is used to study bulk electronic structure of Na0.35CoO2· 1.3H2O, the layered superconductor. In contrast to three-dimensional doped Co oxides, Co 2p core level spectra show well-separated Co3+and Co4+ions. Cluster calculations suggest low-spin Co3+and Co4+character, and a moderate on-site Coulomb correlation energy Udd∼3-5.5 eV. Photon dependent valence band PES identifies Co 3d and O 2p derived states, in near agreement with band-structure calculations.

DOI： 10.1103/PhysRevB.69.180508

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Other Link： http://orcid.org/0000-0002-1124-0451

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

High-energy (hnu=5.95 keV) synchrotron photoemission spectroscopy (PES) is used to study bulk electronic structure of Na0.35CoO2.1.3H(2)O, the layered superconductor. In contrast to three-dimensional doped Co oxides, Co 2p core level spectra show well-separated Co3+ and Co4+ ions. Cluster calculations suggest low-spin Co3+ and Co4+ character, and a moderate on-site Coulomb correlation energy U(dd)similar to3-5.5 eV. Photon dependent valence band PES identifies Co 3d and O 2p derived states, in near agreement with band-structure calculations.

DOI： 10.1103/PhysRevB.69.180508

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DOI： 10.1103/PhysRevB.69.180508

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Ca 2p-3d resonant photoemission spectroscopy of a Cd6Ca crystalline approximant unambiguously demonstrates that the low-lying unoccupied 3d levels of calcium are lowered below the Fermi energy by the formation of the approximant, as suggested from electronic structure calculations [Y Ishii and T Fujiwara, Phys. Rev. Lett. 87, 206408 (2001)]. Moreover, the Ca 3d partial density of states (DOS) obtained near the Fermi energy is in reasonable agreement with theoretical Ca 3d DOS. These results verify the unconventional Picture that the origin of the pseudogap in the Cd-based quasicrystals is due to hybridization of the Ca 3d band with the Cd 5p band.

DOI： 10.1103/PhysRevLett.92.146402

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With a strong help of high-resolution photoemission spectroscopy we demonstrate in this paper that the large thermoelectric power observed in the layered cobalt oxides, such as Ca3Co4O9, Na0.6CoO2, and Bi2Sr2Co2O9, can be well accounted for with the Boltzmann-type metallic electrical conduction. An intense peak with 1.5-2 eV in width was observed in the photoemission spectra with its center at 1.0 eV below the Fermi level EF in these compounds. The density of states at EF is finite but negligibly small at room temperature, because EF is located near the high-energy edge of this narrow band. We calculated thermoelectric power S using the Boltzmann transport equation with the electronic structure near EF determined by the photoemission measurement. The calculated S shows fairly good consistency with the measured value both in its magnitude and the temperature dependence. © 2004 The American Physical Society.

DOI： 10.1103/PhysRevB.69.125410

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Angle-resolved photoemission spectroscopy of Ca(Al0.5Si0.5)(2) (CaAlSi), which is a superconductor (transition temperature is 7.7 K) with the AlB2 structure, revealed that superconducting gaps on two Fermi surfaces (FSs) with three-dimensional character around Gamma(A) and M(L) in the Brillouin zone provide essentially the same superconducting gap value (similar to1.2 meV+/-0.2 meV). This is in contrast to the case of MgB2, in which different FSs exhibit different gap values. The reduced gap value 2Delta(0)/k(B)T(c) of similar to4.2+/-0.2 classifies CaAlSi as a moderately strong-coupling superconductor.

DOI： 10.1103/PhysRevB.69.100506

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

With a strong help of high-resolution photoemission spectroscopy we demonstrate in this paper that the large thermoelectric power observed in the layered cobalt oxides, such as Ca3Co4O9-, Na0.6CoO2, and Bi2Sr2Co2O9, can be well accounted for with the Boltzmann-type metallic electrical conduction. An intense peak with 1.5-2 eV in width was observed in the photoemission spectra with its center at 1.0 eV below the Fermi level E-F in these compounds. The density of states at E-F is finite but negligibly small at room temperature, because EF IS located near the high-energy edge of this narrow band. We calculated thermoelectric power S using the Boltzmann transport equation with the electronic structure near EF determined by the photoemission measurement. The calculated S shows fairly good consistency with the measured value both in its magnitude and the temperature dependence.

DOI： 10.1103/PhysRevB.69.125410

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DOI： 10.1103/PhysRevB.70.159902

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X-ray photoemission spectroscopy is used to compare the electronic structures of MgB2 and ZrB2. The B 1s core levels in high-quality MgB2 and ZrB2 exhibit a single asymmetric peak typical of a metallic boride. The Mg 2p core level shows a single peak with negligible intensity in charge-transfer satellites and no correlation effects. The Mg 2p and B 1s core-level spectra exhibit a broad bulk plasmon feature centered at about 22 eV from the main peak, in good accord with calculations. The measured valence bands are consistent with band-structure calculations indicating a higher density of states (DOS) at EF for MgB2 compared to ZrB2. The high Tc in MgB2 is due to p-derived DOS, while ZrB2 is dominated by d-derived states at EF. © 2003 The American Physical Society.

DOI： 10.1103/PhysRevB.68.174506

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

X-ray photoemission spectroscopy is used to compare the electronic structures of MgB2 and ZrB2. The B 1s core levels in high-quality MgB2 and ZrB2 exhibit a single asymmetric peak typical of a metallic boride. The Mg 2p core level shows a single peak with negligible intensity in charge-transfer satellites and no correlation effects. The Mg 2p and B 1s core-level spectra exhibit a broad bulk plasmon feature centered at about 22 eV from the main peak, in good accord with calculations. The measured valence bands are consistent with band-structure calculations indicating a higher density of states (DOS) at E-F for MgB2 compared to ZrB2. The high T-c in MgB2 is due to p-derived DOS, while ZrB2 is dominated by d-derived states at E-F.

DOI： 10.1103/PhysRevB.68.174506

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We have performed soft X-ray emission (SXE) and high-resolution photoemission (PE) spectroscopy to study the electronic structures of beta-HfNCl and Na-intercalated HfNCl. Both the PE and SXE spectra of Na0.37HfNCl show an additional structure within the band gap of beta-HfNCl, without an apparent shift in the energy position of valence-band features. These results clearly reveal intercalant-induced changes in the electronic structure of the layered nitride chlorides. From these results, we propose non-rigid band like behavior for this system in contrast to generally expected rigid-band filling upon intercalation. (C) 2003 Elsevier B.V. All rights reserved.

DOI： 10.1016/S0921-4534(03)00752-4

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We have performed X-ray absorption and valence band photoemission spectroscopy on ZrB2, which has the same AlB2 structure of MgB2, to study the electronic structure. From X-ray absorption spectra near the B 1s-2p threshold, we found a small absorption edge at 188.23 eV with a negligible angular dependence upto 190 eV photon energy. This indicates that B 2p partial density of states near the Fermi level (E-F) is three dimensional, in sharp contrast to that of MgB2. Resonant photoemission study across the B 1s-2p threshold was found to be difficult due to a large contribution of Auger electrons. On the other hand, resonant photoemission across the Zr 3p-4d threshold does show small change in intensity within 4 eV of EF. This indicates that the occupied states near EF have substantial Zr 4d character. These observations are in relatively good agreement with band structure calculations. (C) 2003 Elsevier B.V. All rights reserved.

DOI： 10.1016/S0921-4534(03)00947-X

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The superconducting-gap of MgB2 has been studied by high-resolution angle-resolved photoemission spectroscopy. The results show that superconducting gaps with values of 5.5 and 2.2 meV open on the sigma band and the pi band, respectively, but both the gaps close at the bulk transition temperature, providing a definitive experimental evidence for the two-band superconductivity with strong interband pairing interaction in MgB2. The experiments validate the role of k-dependent electron-phonon coupling as the origin of multiple-gap superconductivity as well as the high transition temperature of MgB2.

DOI： 10.1103/PhysRevLett.91.127001

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Soft x-ray energy-dependent angle-resolved photoemission measurements are performed on the heavy-fermion superconductor CeIrIn5 to study the Fermi surface (FS) and the electronic structure near the Fermi level in the GammaMZA plane. The non-dispersive band designated as the Cc 4f(7/2)(1) final state is found at 280 meV along with the main body of the valence band consisting of highly dispersive In 5p bands, which are in good qualitative agreement with the band structure calculation based on the itinerant 4f electron model. Furthermore, we found two electron-like and one hole-like cylindrical FSs at the M (A) and Gamma (Z) points. The Fermi momenta of the observed FSs are consistent with the de Haas-van Alphen experiment, while they contradict the stronger anisotropy of the FS along the k(z) direction predicted by the band calculation. The results suggest stronger two-dimensionality of the electronic structure of CeIrIn5 and point to the need for its theoretical verification.

DOI： 10.1088/0953-8984/15/28/340

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

We study the superconducting electronic structures of Nb3Al and MgB2 using high-resolution spectroscopy. The obtained spectrum of Nb3Al measured below T-c shows clear opening of the superconducting gap with a sharp pile up in the density of states and a shift of the leading edge. In addition, the spectrum shows a peak-dip-hump line shape expected from the strong-coupling theory. On the other hand, for MgB2, the superconducting-state spectrum measured at 5.4 K shows a coherent peak with a shoulder structure, in sharp contrast to that expected from a single isotropic gap. The superconducting spectral shape of MgB2 can be explained in terms of a multicomponent gap. (C) 2002 Elsevier Science Ltd. All rights reserved.

DOI： 10.1016/S0022-3697(02)00236-6

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DOI： 10.1016/S0022-3697(02)00019-7

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We report photoemission study of the pyrochlore superconductor Cd2Re2O7. The valence-band spectra show the Re 5d dominant band close to the Fermi level (EF), consistent with the recent band calculations. Temperature-dependent study of the Re 5d band reveals a gradual change in spectral shape, as is seen from a gradual modification of difference spectra below 200 K. These results are related to another transition at 200 K reported from resistivity and susceptibility measurements and indicate that the transition involves a change in electronic structures of the Re 5d band near EF.

DOI： 10.1103/PhysRevB.66.012516

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Superconducting transitions of elemental metals and Ni borocarbides are studied using ultrahigh-resolution and low-temperature photoemission spectroscopy. Photoemission spectra of elemental metals (Nb and Pb) show the appearance of a sharp peak just below the Fermi level and a shift of the leading edge, indicating clear opening of the superconducting gap. Furthermore, Pb spectrum shows a dip and hump structure beyond the peak, characteristic of the strong-coupling superconductivity. On the other hand, comparative study of Y(Ni1-xPtx)(2)B2C (x=0.0 and 0.2) shows a difference in the slope of the leading edge, which can be explained in terms of a highly anisotropic s-wave gap in YNi2B2C, consistent to magnetic field dependent specific heat measurements. These observations demonstrate that photoemission spectroscopy is a useful and reliable technique to study electronic structures and phase transitions at very small energy scales. (C) 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0368-2048(02)00047-6

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMERICAN PHYSICAL SOC  

We report photoemission study of the pyrochlore superconductor Cd2Re2O7. The valence-band spectra show the Re 5d dominant band close to the Fermi level (E-F), consistent with the recent band calculations. Temperature-dependent study of the Re 5d band reveals a gradual change in spectral shape, as is seen from a gradual modification of difference spectra below 200 K. These results are related to another transition at 200 K reported from resistivity and susceptibility measurements and indicate that the transition involves a change in electronic structures of the Re 5d band near E-F.

DOI： 10.1103/PhysRevB.66.012516

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：BLACKWELL MUNKSGAARD  

High-resolution HeI photoemission spectroscopy (UPS), Mn 2p-3d resonant photoemission spectroscopy (RPES) and Mn 2p X-ray absorption spectroscopy (XAS) have been performed to investigate the electronic structure and its effect on the electrical resistivity in (La1-zNdz)(0.46)Sr0.54MnO3 (z = 0, 0,2, 0.6 and 1.0). It was found that in the UPS and RPES spectra the Fermi edge persisted over the temperature range 15 less than or equal to T less than or equal to 340 K regardless of the magnetic structure or the composition of the samples. The density of states at the Fermi level [N(E-F)] in the samples where 0 less than or equal to z less than or equal to 0.6 was increased drastically at the Curie temperature (T-C) with decreasing temperature, but essentially kept unchanged across the Neel temperature (T-N). A fairly large reduction at T-C and a small increase at T-N in the electrical resistivity with decreasing temperature are found to be well accounted for in terms of the temperature dependence of N(E-F). The presence of a finite N(E-F) in the insulating Nd0.46Sr0.54MnO3 was also found. Thus the origin of the insulating behavior in this sample can be regarded as the Anderson localization associated with the small density of states and the chemical disorder between Nd and Sr.

DOI： 10.1107/S0909049502008889

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The electronic structure of Heusler-type Fe2VAl has been studied by high-resolution photoelectron spectroscopy with the excitation photon energy hnu ranging from 21.2 eV (the He I laboratory light source) to 904 eV (the soft X-ray synchrotron light source) for clean surfaces prepared by scraping or fracturing polycrystalline and single crystalline specimens. Photoelectron spectra recorded for the fractured surfaces show a 10 eV-wide valence band with fine structures and a clear decrease in the intensity towards the Fermi level E-F, while a high intensity at EF and no fine structures are observed for the scraped surface. Comparison with the theoretical density of states (DOS) indicates that the vacuum ultraviolet photoelectron spectra emphasize the transition-metal 3d bands but the soft X-ray photoelectron spectra agree remarkably well with the DOS including the fine structures and the pseudogap at EF. The present results suggest that the electronic structure of Fe2VAl is highly sensitive to possible strain and defects induced by scraping. Bulk electronic structures of Fe2VAl are discussed in relation to the reported fascinating transport properties.

DOI： 10.1107/S0909049502009214

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Low-temperature ultrahigh-resolution photoemission (PE) spectroscopy of the recently discovered binary icosahedral (i) Cd5.7Yb and its 1/1-cubic approximant Cd6Yb has commonly revealed a sharp Fermi edge with a marked spectral intensity decrease towards the Fermi level (EF), indicating the presence of a dip near EF which is consistent with the recent result of electronic structure calculations [Y. Ishii and T. Fujiwara, Phys. Rev. Lett. 87, 206408 (2001)], and the dip at EF is found to be much deeper in i-Cd5.7Yb than in 1/1-Cd6Yb. Moreover, surface core-level shifts (SCS's) are clearly observed in the quasicrystal and the approximant, and the SCS's are found to be substantially different between these two compounds of different long-range orders.

DOI： 10.1103/PhysRevB.65.224207

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Low-temperature ultrahigh-resolution photoemission (PE) spectroscopy of the recently discovered binary icosahedral (i) Cd5.7Yb and its 1/1-cubic approximant Cd6Yb has commonly revealed a sharp Fermi edge with a marked spectral intensity decrease towards the Fermi level (E-F), indicating the presence of a dip near E-F which is consistent with the recent result of electronic structure calculations [Y. Ishii and T. Fujiwara, Phys. Rev. Lett. 87, 206408 (2001)], and the dip at E-F is found to be much deeper in i-Cd5.7Yb than in 1/1-Cd6Yb. Moreover, surface core-level shifts (SCS's) are clearly observed in the quasicrystal and the approximant, and the SCS's are found to be substantially different between these two compounds of different long-range orders.

DOI： 10.1103/PhysRevB.65.224207

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We have performed angle-resolved photoemission spectroscopy of mixed valence oxide V6O13 that shows a metal-insulator transition (MIT) at ≃ 150 K. In the metallic phase, we observe two bands near the Fermi level (EF). One is a prominent band located around 0.8 eV and the other is a weak structure around 0.2 eV that shows dispersion toward EF only along the b axis. Furthermore, though the momentum distribution curve at EF shows a peak at kb/π=0.29±0.01 indicative of a EF crossing, the intensity of the band near EF is strongly suppressed in the region of EF. These observations indicate quasi-one-dimensional electronic states of V6O13, consistent with a highly anisotropic behavior observed from resistivity and optical conductivity measurements. Across the MIT, the band near EF shifts to higher binding energy and becomes less dispersive, resulting in opening of an energy gap of 0.2 eV. We discuss some implications of the experimental results.

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We have performed angle-resolved photoemission spectroscopy of mixed valence oxide V6O13 that shows a metal-insulator transition (MIT) at similar or equal to150 K. In the metallic phase, we observe two bands near the Fermi level (E-F). One is a prominent band located around 0.8 eV and the other is a weak structure around 0.2 eV that shows dispersion toward E-F only along the b axis. Furthermore, though the momentum distribution curve at E-F shows a peak at kb/pi=0.29+/-0.01 indicative of a E-F crossing, the intensity of the band near E-F is strongly suppressed in the region of E-F. These observations indicate quasi-one-dimensional electronic states of V6O13, consistent with a highly anisotropic behavior observed from resistivity and optical conductivity measurements. Across the MIT, the band near E-F shifts to higher binding energy and becomes less dispersive, resulting in opening of an energy gap of 0.2 eV. We discuss some implications of the experimental results.

DOI： 10.1103/PhysRevB.65.205124

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We study the electronic structure of single-crystal "black" semiconducting SmS using x-ray absorption and resonance photoemission spectroscopy across the 4d-4f threshold and high-resolution temperature-dependent valence-band photoemission spectroscopy. The 4d-4f on-resonance spectra show mixed valency of Sm2+ and Sm3+ states in semiconducting SmS at low temperature (T = 30 K). The high-resolution spectra show a pseudogap within 20 meV of the Fermi level at low temperatures. This pseudogap is gradually filled up accompanied by a redistribution of spectral weight over a larger energy scale of ∼200 meV on increasing temperature, resulting in an incipient metallic phase at room temperature. The two energy scales can be associated with the coherence temperature Tc and the Kondo temperature TK, respectively. The changes in the single-particle density of states at and about the Fermi level are compatible with recent theoretical results on "exhaustion physics" in the periodic Anderson model. The present study indicates a Kondo lattice picture for SmS and suggests a relation between the mixed valency and the temperature dependence of the pseudogap for "black" semiconducting SmS.

DOI： 10.1103/PhysRevB.65.155201

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We study the electronic structure of single-crystal "black" semiconducting SmS using x-ray absorption and resonance photoemission spectroscopy across the 4d-4f threshold and high-resolution temperature-dependent valence-band photoemission spectroscopy. The 4d-4f on-resonance spectra show mixed valency of Sm2+ and Sm3+ states in semiconducting SmS at low temperature (T=30 K). The high-resolution spectra show a pseudogap within 20 meV of the Fermi level at low temperatures. This pseudogap is gradually filled up accompanied by a redistribution of spectral weight over a larger energy scale of similar to200 meV on increasing temperature, resulting in an incipient metallic phase at room temperature. The two energy scales can be associated with the coherence temperature T-c and the Kondo temperature T-K, respectively. The changes in the single-particle density of states at and about the Fermi level are compatible with recent theoretical results on "exhaustion physics" in the periodic Anderson model. The present study indicates a Kondo lattice picture for SmS and suggests a relation between the mixed valency and the temperature dependence of the pseudogap for "black" semiconducting SmS.

DOI： 10.1103/PhysRevB.65.155201

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We study the superconducting gap of MgB2 using high-resolution photoemission. In the spectrum measured at 5.4 K, a coherent peak with a shoulder structure is observed. We find that a simulation using two Dynes functions with the gap sizes of 1.7 and 5.6 meV reproduces the superconducting-state spectrum better than that using a single Dynes function and anisotropic Dynes functions. We also find that both of the smaller and larger gap close at the bulk transition temperature. These results indicate a multiple superconducting gap of MgB2. (C) 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4526(01)01282-0

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We have studied the Fermi surface (FS) and superconducting gap of 2H-NbSe2, using angle-resolved photoemission spectroscopy with an energy resolution of 2.2 meV and an angle resolution of +/-0.13degrees. We succeeded in distinguishing all FS sheets predicted from band calculations and found that the observed hexagonal Fermi surface nesting vector centered at the Gamma point is larger than the CDW nesting vector reported from neutron diffraction measurements. Furthermore, a superconducting gap at one FS sheet is successfully observed. (C) 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4526(01)01293-5

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We study the electronic properties of quasi-one-dimensional V6O13 which exhibits a metal-insulator transition using angle-resolved photoemission spectroscopy (ARPES), optical conductivity, and soft X-ray Raman spectroscopy (SXRS). From ARPES, we observe an energy gap of 0.2eV across the metal-insulator transition with change in the lower Hubbard band. Band dispersion is found only along the b-axis, consistent with highly anisotropic behavior observed in optical conductivity and SXRS measured on the same samples. These results indicate the electronic properties of V6O13 are dominated by b-axis electronic structure. (C) 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4526(01)01186-3

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Ultrahigh-resolution photoemission spectra of CeRu4Sb12 reveal crossover behavior into a low carrier state. The density of states (DOS) around the Fermi level (EF) decreases on lowering temperature and the semimetalliclike DOS is observed at 5.9 K. The temperature dependence of DOS at EF is explained in terms of the development of the coherence. The smaller hybridization strength between the Ru-d state and the Ce-4f state prevents the hybridization gap from opening in CeRu4Sb12, which is realized in CeRu4P12.

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Ultrahigh-resolution photoemission spectra of CeRu4Sb12 reveal crossover behavior into a low carrier state. The density of states (DOS) around the Fermi level (E-F) decreases on lowering temperature and the semimetalliclike DOS is observed at 5.9 K. The temperature dependence of DOS at E-F is explained in terms of the development of the coherence. The smaller hybridization strength between the Ru-d state and the Ce-4f state prevents the hybridization gap from opening in CeRu4Sb12, which is realized in CeRu4P12.

DOI： 10.1103/PhysRevB.65.041105

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Ultrahigh-resolution photoemission spectra of (formula presented) reveal crossover behavior into a low carrier state. The density of states (DOS) around the Fermi level (formula presented) decreases on lowering temperature and the semimetalliclike DOS is observed at 5.9 K. The temperature dependence of DOS at (formula presented) is explained in terms of the development of the coherence. The smaller hybridization strength between the Ru- (formula presented) state and the (formula presented) state prevents the hybridization gap from opening in (formula presented) which is realized in (formula presented). © 2002 The American Physical Society.

DOI： 10.1103/PhysRevB.65.041105

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER ASSOC ADVANCEMENT SCIENCE  

High-resolution angle-resolved photoemission spectroscopy was used to study the superconducting energy gap and changes in the spectra[ function across the superconducting transition in the quasi-two-dimensional superconductor 2H-NbSe2. The momentum dependence of the superconducting gap was determined on different Fermi surface sheets. The results indicate Fermi surface sheet-dependent superconductivity in this Low-transition temperature multiband system and provide a description consistent with thermodynamic measurements and the anomalous de Haas-van Alphen oscillations observed in the superconducting phase. The present data suggest the importance of Fermi surface sheet-dependent superconductivity in explaining exotic superconductivity in other multiband systems with complex Fermi surface topology, such as the borides and f-electron superconductors.

DOI： 10.1126/science.1065068

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We study the single-particle density of states (DOS) across the superconducting transition (Tc=31 K) in single-crystal Ba0.67K0.33BiO3 using ultrahigh resolution angle-integrated photoemission spectroscopy. The superconducting gap opens with a pileup in the DOS, Δ(5.3 K) = 5.2 meV and 2Δ(0)/kBTc = 3.9. In addition, we observe a pseudogap below and above Tc, occurring as a suppression in intensity over an energy scale up to the breathing mode phonon (∼70 meV). The results indicate electron-phonon coupling induces a pseudogap in Ba0.67K0.33BiO3.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society  

We study the electronic structure and superconducting transition of silicon clathrate Ba8Si46 (Tc=8 K) using photoemission spectroscopy. We observe a narrow band at the Fermi level (EF), whose width (∼0.3 eV) is substantially smaller than that of band structure calculations (∼1.5 eV). Ultrahigh-resolution measurements show a superconducting gap at 5.4 K [2Δ(0)/kBTc=3.51]. Fine structures associated with phonons are observed within 70 meV of EF. These results characterize Ba8Si46 as a weak-coupling superconductor most probably driven by phonon.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We study the single-particle density of states (DOS) across the superconducting transition (T-c = 31 K) in single-crystal Ba0.67K0.33BiO3 using ultrahigh resolution angle-integrated photoemission spectroscopy. The superconducting gap opens with a pileup in the DOS, Delta (5.3 K) = 5.2 meV and 2 Delta (0)/k(B)T(c) = 3.9. In addition, we observe a pseudogap below and above T-c occurring as a suppression in intensity over an energy scale up to the breathing mode phonon (similar to 70 meV). The results indicate electron-phonon coupling induces a pseudogap in Ba0.67K0.33BiO3.

DOI： 10.1103/PhysRevB.64.180509

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We study the electronic structure and superconducting transition of silicon clathrate Ba8Si46 (T-c = 8 K) using photoemission spectroscopy. We observe a narrow band at the Fermi level (E-F), whose width (similar to0.3 eV) is substantially smaller than that of band structure calculations (similar to1.5 eV). Ultrahigh-resolution measurements show a superconducting gap at 5.4 K [2 Delta (0)/k(B)T(c)=3.51]. Fine structures associated with phonons are observed within 70 meV of E-F. These results characterize Ba8Si46 as a weak-coupling superconductor most probably driven by phonon.

DOI： 10.1103/PhysRevB.64.172504

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We use photoemission (PE) and x-ray absorption (XA) spectroscopy to study intercalation-induced changes in the electronic structure of beta -HfNCl and Na0.22HfNCl. PE shows appearance of new states in Na0.22HfNCl within the gap of beta -HfNCl, with negligible changes in the energy positions of valence-band features of the parent, suggesting a non-rigid-band behavior. N 1 s,XA results show that the tail of the conduction band, with a quasi-two-dimensional character, extends to lower energy in Na0.22HfNCl than in beta -HfNCl. The study provides information on the electronic structure and its change across the insulator-to-metal transition of the. new class of superconductors.

DOI： 10.1103/PhysRevB.64.153107

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We study the new binary intermetallic superconductor MgB2 using high-resolution photoemission spectroscopy. The superconducting-state spectrum measured at 5.4 K shows a coherent peak with a shoulder structure, in sharp contrast to that expected from a simple isotropic-gap opening. The spectrum can be well reproduced using the weighted sum of two Dynes functions with the gap sizes of 1.7 and 5.6 meV. Temperature-dependent study shows that both gaps close at the bulk transition temperature. These results provide spectroscopic evidence for a multiple gap of MgB2.

DOI： 10.1103/PhysRevLett.87.177006

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We study the normal-state electronic excitations probed by angle-resolved photoemission spectroscopy (ARPES) in Bi1.6Pb0.4Sr2CuO6 (Bi2201) and Bi2Sr2CaCu2O8 divided by delta (Bi2212). Our main goal is to establish explicit criteria for determining the Fermi surface from ARPES data on strongly interacting systems where sharply defined quasiparticles do not exist and the dispersion is very weak in parts of the Brillouin zone. Additional complications arise from strong matrix element variations within the zone. We present detailed results as a function of incident photon energy, and show simple experimental tests to distinguish between an intensity drop due to matrix element effects and spectral weight loss due to a Fermi crossing. We reiterate the use of polarization selection rules in disentangling the effect of umklapps due to the BiO superlattice in Bi2212. We conclude that, despite all the complications, the Fermi surface can be determined unambiguously; it is a single large hole barrel centered about (pi,pi) in both materials.

DOI： 10.1103/PhysRevB.63.224516

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We report the doping level dependence of the band dispersion and the shape of the Fermi surface for Pb-doped Bi2Sr2CuO6+delta superconductor by use of angle resolved photoemission spectroscopy with extremely high angular and energy resolutions. It is found that the band dispersion of Bi2Sr2CuO6+delta is fairly rigid regardless of its doping level in the over-doped range, and that it shifts toward higher binding energy with decreasing doping. We also found that one of the heavily over-doped samples possesses an electron-like Fermi surface centered at (0,0) point, while less doped samples show a hole-like Fermi surface. This is the first observation of the transformation between electron-like and hole-like Fermi surfaces with varying the doping level in Bi2Sr2CuO6+delta. (C) 2001 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0368-2048(00)00354-6

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We have performed ultra-high resolution photoemission spectroscopy of simple metals, Nb (T-c=9.26 K) and Pb (T-c=7.19 K) in order to study the changes in the density of states across the transition temperatures. The spectra of both compounds measured at 5.3 K show an opening of the superconducting gap accompanied by a leading-edge shift and an appearance of a sharp peak just below the Fermi level. Furthermore, the ph spectrum clearly shows peak-dip-hump structures due to strong electron-phonon coupling, as seen in the high-T-c cuprates. This makes it a characteristic of strong-coupling superconductivity. (C) 2001 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0368-2048(00)00366-2

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We carried out ultrahigh-resolution photoemission (URPE) and resonant-inverse photoemission spectroscopy on CePd3 and CePd2.9Cu0.1 compounds. The URPE spectra of CePd3 and CePd2.9Cu0.1 clearly show small humps in the vicinity of Fermi level well below the Kondo temperature. The hump is gradually formed below the characteristic coherent temperature T*. We conclude that it gives direct evidence of the appearance of the small pseudogap below T* in CePd3 and CePd2.9Cu0.1. (C) 2001 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0368-2048(00)00333-9

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Ultrahigh-resolution (DeltaE = 7 meV angle-resolved (ARPES) and angle-integrated (AIPES) photoemission spectroscopy have been performed on optimally doped Bi2Sr2CaCu2O8 (Bi2212) and La1.85Sr0.15CuO4 (LSCO). The ARPES spectrum of Bi2212 at the (pi, 0)-(pi, pi) crossing shows a pseudogap at EF above T-c, which is smoothly connected from the superconducting gap across T-c and closes slightly above T-c(T-* = 110 K). On the other hand, the AIPES spectra both in Bi2212 and LSCO show a remarkable depletion of the density of states in a relatively wider energy (100-150 meV) region near EF The temperature-induced filling of the depletion persists far above T-c and T-*. This suggests that there are two different pseudogaps in the high-T-c superconductors: one is likely associated with pairing and the other is attributed to the development of antiferromagnetic correlation. (C) 2000 Elsevier Science Ltd. All rights reserved.

DOI： 10.1016/S0022-3697(00)00098-6

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

We have performed a systematic doping-dependent angle-resolved photoemission spectroscopy (ARPES) of Pb-substituted Bi2201 (Bi1.80Pb0.38Sr2.01CuO6-delta) With high energy (7-11 meV) and momentum (0.01 Angstrom (-1)) resolutions. We found that all the samples show a large hole-like Fermi surface centered at (pi,pi), whose volume changes systematically as a function of doping. The superconducting gap exhibits a d(x2-y2)-like anisotropy with a typical gap value of 10-15 meV near (pi ,0). By comparing with Bi2Sr2CaCu2O8 (Bi2212), we found that the size of superconducting gap is well scaled with the maximum T-c (T-c(max)) Doping-dependence of spectral line shape at (pi ,0) is very similar between Bi2201 and Bi2212, while its energy scale is different by a factor of 4, corresponding to the ratio of T-c(max) between the two compounds. These indicate that the superconducting and pseudogap properties are characterized by T-c(max) irrespective of the number of CuO2 layers or the BiO superstructure. (C) 2000 Elsevier Science Ltd. All rights reserved.

DOI： 10.1016/S0022-3697(00)00119-0

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We study the normal-state electronic excitations probed by angle-resolved photoemission spectroscopy (ARPES) in Bi1.6Pb0.4Sr2CuO6 (Bi2201) and Bi2Sr2CaCu2O8+δ (Bi2212). Our main goal is to establish explicit criteria for determining the Fermi surface from ARPES data on strongly interacting systems where sharply defined quasiparticles do not exist and the dispersion is very weak in parts of the Brillouin zone. Additional complications arise from strong matrix element variations within the zone. We present detailed results as a function of incident photon energy, and show simple experimental tests to distinguish between an intensity drop due to matrix element effects and spectral weight loss due to a Fermi crossing. We reiterate the use of polarization selection rules in disentangling the effect of umklapps due to the BiO superlattice in Bi2212. We conclude that, despite all the complications, the Fermi surface can be determined unambiguously
it is a single large hole barrel centered about (π, π) in both materials.

DOI： 10.1103/PhysRevB.63.224516

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The superconducting gaps (SGs) of YNi2B2C and Y(Ni0.8Pt0.2)2CB2C were studied using ultrahigh-resolution photoemission (PE) spectroscopy. A small but important difference in spectral shape was observed and attributed to the change in anisotropy of the SG between x=0.0 and 0.2 compounds.

DOI： 10.1103/PhysRevLett.85.4952

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier Science Publishers B.V.  

The hole-state and defect structure of proton conductor SrTi1-xScxO3 have been studied by high-resolution X-ray absorption spectroscopy. The proton-induced level is observed in proton-doped SrTi1-xScxO3. The energy separation between the top of the valence band and the bottom of the acceptor or proton level is considered to be the activation energy determined by electronic or proton conductivity.

DOI： 10.1016/S0167-2738(00)00319-2

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We report an ultrahigh-resolution (Delta E similar to 8 meV) temperature-dependent photoemission spectroscopy on the Kondo insulators CeRhAs and CeRhSb. We have observed a pseudogap at the Fermi level at low temperature for both compounds, which is gradually filled-in with increasing temperature. We found that, the, size of pseudogap is well scaled with the Kondo temperature (T-K) while the temperature evolution is dominated by another characteristic temperature much lower than T-K. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4526(99)00827-3

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：KLUWER ACADEMIC/PLENUM PUBL  

We performed angle-resolved photoemission spectroscopy (ARPES) on Sr2CuO3 and Sr14Cu24O41 to study the difference of electronic structure between the corner-sharing one-dimensional (1D)chain and the two-leg ladder cuprate. ARPES spectra of Sr2CuO3 show two independent dispersive bands near the Fermi level (E-F), which are ascribed to the spinon and the holon band due to the spin-charge separation in the ID chain. In contrast, ARPES spectra of Sr14Cu24O41 exhibit only one dispersive band near E-F With the periodicity of ladder sublattice, which is assigned to a quasi-particle band where holon and spinon are confined together owing to the opening of the spin gap on the ladder. The present results show a clear difference in the nature of the electronic structure near E-F between the Tomonaga-Luttinger liquid (Sr2CuO3) and the Luther-Emery liquid (Sr14Cu24O41).

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We study the changes in the electronic structure associated with low-Tc strong-coupling superconducting transitions in Pb and Nb using ultrahigh-resolution (2.3 meV) temperature-dependent (5.3-12.0 K) photoemission spectroscopy. We observe peaks in the density of states on entering the superconducting-phase accompanying gap formation and spectacular redistribution of spectral weight at low energy scales as a function of temperature. The well-known peak-dip feature of the high-Tc cuprates is seen in Pb, making it a characteristic of strong-coupling superconductivity. © 2000 The American Physical Society.

DOI： 10.1103/PhysRevLett.85.1966

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The electronic structure of lightly Nb-doped (Formula presented) has been investigated using high resolution x-ray absorption spectroscopy (XAS). Below the O (Formula presented) threshold, XAS spectra show two features due to empty states whose energy positions match with those of the (Formula presented) photoemission spectra in the band gap energy region of the parent undoped (Formula presented) Similar features are also observed in lightly La-doped (Formula presented) These features exhibit systematic temperature dependence, which is well explained by the Fermi-Dirac distribution function. This indicates that the two features in the band gap are real bulk states such as simple donor states. © 2000 The American Physical Society.

DOI： 10.1103/PhysRevB.61.12860

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier Science Publishers B.V.  

We have performed a systematic high energy (7-11 meV) and momentum (0.01 angstrom-1) resolution angle-resolved photoemission spectroscopy (ARPES) on superstructure-free Bi1.80Pb0.38Sr2.01CuO6-δ with various dopings. All the samples from overdoped Tc = 0K to underdoped Tc = 18K possess a large hole-like Fermi surface centered at X point, whose volume shows a systematic evolution with doping. The superconducting gap exhibits a dx(2)-y(2)-like anisotropy with a typical gap value of 10-15 meV. By comparing with Bi2Sr2CaCu2O8, we found that the size of superconducting gap and the ARPES spectral feature near EF are well scaled with the maximum Tc (Tc max). This suggests that the superconducting properties are characterized by Tc max irrespective of the number of CuO2 layers or the structural modulation in BiO layer.

DOI： 10.1016/S0921-4534(00)01099-6

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Language：English   Publisher：The Japan Society of Applied Physics  

The Ti-O hybridization effect on the ferroelectric phase transition of BaTiO3 has been studied by resonant-photoemission spectroscopy. The Ti 3d partial-density-of-state in the valence band of the paraelectric phase is larger than that of the ferroelectric phase. This fact indicates that the Ti–O hybridization effect is stronger in the paraelectric phase.

DOI： 10.1143/JJAP.38.5667

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High-resolution photoemission spectroscopy has been performed on CeRhAs and CeRhSb to study the temperature-induced evolution of the “Kondo-insulator” gap. We have observed that a pseudogap opens at EF at low temperatures while it is gradually filled with increasing temperature. The size of the pseudogap is well scaled with the Kondo temperature (TK) of each compound while the temperature evolution is dominated by another characteristic temperature Tcoh (&lt
TK). The results are qualitatively consistent with the theoretical prediction from the Anderson lattice model. © 1999 The American Physical Society.

DOI： 10.1103/PhysRevLett.82.1943

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The resonant inelastic x-ray scattering spectrum in insulating cuprates is examined by using the exact diagonalization technique on small clusters in the two-dimensional Hubbard model with second and third neighbor hopping terms. When the incident photon energy is tuned near the Cu K absorption edges, we find that the features of the unoccupied upper Hubbard band can be extracted from the spectrum through an anisotropic momentum dependence. They provide an opportunity for the understanding of the different behavior of hole- and electron-doped superconductors. © 1999 The American Physical Society.

DOI： 10.1103/PhysRevLett.83.3709

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We have performed an ultrahigh-resolution \\(ΔE˜7 meV\\) photoemission spectroscopy on La1.85Sr0.15CuO4 \\(Tc = 38 K\\) at 16-250 K. We found a pseudogap at EF with the size of 30-35 meV, which is gradually filled in by a transfer of states from the higher energy region with increasing temperature, but persists over 200 K. The gap energy is consistent with the characteristic temperatures of the magnetic susceptibility and the Hall coefficient. The temperature evolution is well scaled with that of the Hall coefficient. The pseudogap is not smoothly connected to the superconducting gap \\(Δ˜8 meV\\) below Tc, suggesting a different origin from that in Bi2Sr2CaCu2O8. © 1999 The American Physical Society.

DOI： 10.1103/PhysRevLett.83.2254

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Comparing photoemission measurements on Bi2212 with penetration depth data, we show that a description of the nodal excitations of the d-wave superconducting state in terms of noninteracting quasiparticles is inadequate, and we estimate the magnitude and doping dependence of the Landau interaction parameter which renormalizes the linear T contribution to the superfluid density. Furthermore, although consistent with d-wave symmetry, the gap with underdoping cannot be fit by the simple coskx−cosky form, which suggests an increasing importance of long range interactions as the insulator is approached. © 1999 The American Physical Society.

DOI： 10.1103/PhysRevLett.83.840

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We report angle-resolved photoemission spectroscopy (ARPES) on a single Cu-O chain compound Sr2CuO3 with Ne I photons. In contrast with previous ARPES with higher-energy photons, the present result clearly shows two well-separated branches of dispersions near EF ascribable to spinon and holon excitations, respectively. This indicates that spin and charge degrees of freedom are separated in one-dimensional Cu-O chains, supporting the theoretical prediction as well as the previous ARPES interpretation. Comparison with previous ARPES results indicates that surface, photoionization cross section, and finite-temperature effects substantially modify ARPES spectra. © 1999 ThAmerican Physical Society.

DOI： 10.1103/PhysRevB.59.7358

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Publishing type：Research paper (scientific journal)  

We use the gradient of the energy-integrated angle resolved photoemission (ARPES) intensity in order to define precisely the Fermi surface (FS) in BSCCO superconductors. We show that, independent of the photon energy, the FS is a hole barrel centered at (π, π). Then, the superconducting gap along the FS is precisely determined from ARPES measurements on overdoped and underdoped samples of Bi2212. As the doping decreases, the maximum gap increases, but the slope of the gap near the nodes decreases. Though consistent with d-wave symmetry, the gap with underdoping cannot be fit by the simple cos(k ) - cos(k ) form. A comparison of our ARPES results with available penetration depth data indicates that the renormalization of the linear T suppression of the superfluid density at low temperatures due to quasiparticle excitations around the d-wave nodes is large and doping dependent. x y

DOI： 10.1023/A:1022569616840

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The change of the electronic structure across the metal-insulator transition in a Mott-Hubbard system (Formula presented) has been investigated by x-ray absorption and photoemission spectroscopy. The results are compared with (Formula presented) where hole doping is achieved by excess oxygen instead of Sr substitution. It is found that additional doped-hole states are created in the insulator gap for both cases, while the density of states at the Fermi level in (Formula presented) is nearly half of that in (Formula presented) at the same nominal doping. This suggests a strong reduction in the mobility of carriers due to cation vacancies produced by excess oxygen. © 1999 The American Physical Society.

DOI： 10.1103/PhysRevB.59.1815

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Macmillan Magazines Ltd  

The Fermi surface - the set of points in momentum space describing gapless electronic excitations - is a central concept in the theory of metals. In this context, the normal 'metallic' state of the optimally doped high-temperature superconductors is not very unusual: above the superconducting transition temperature, T(c), there is evidence for a large Fermi surface1-3 despite the absence of well-defined elementary excitations. In contrast, the normal state of underdoped high-temperature superconductors differs in that there is evidence for a 'pseudogap' above T(c) (refs 4-6). Here we examine, using angle-resolved photoemission spectroscopy, the temperature dependence of the Fermi surface in underdoped Bi2Sr2CaCu2O(8+δ). We find that, on cooling the sample, the pseudo-gap opens up at different temperatures for different points in momentum space. This leads to an initial breakup of the Fermi surface, at a temperature T*, into disconnected arcs, which then shrink with decreasing temperature before collapsing to the point nodes of the superconducting ground state below T(c). This unusual behaviour, where the Fermi surface does not form a continuous contour in momentum space as in conventional metals, is unprecedented in that it occurs in the absence of long-range order. Moreover, although the superconducting gap below T(c) evolves smoothly into the pseudogap above T(c), the pseudogap differs in its unusual temperature-dependent anisotropy, implying an intimate but non-trivial relationship between the pseudo-gap and the superconducting gap.

DOI： 10.1038/32366

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier Ltd  

Angle-resolved photoemission measurements were performed for one-dimensional Sr2CuO3 which consists of single Cu-O chains. Two dispersive bands were observed near EF in the first half of the Brillouin zone (k = 0 to π/2) of the chain-direction. Both bands have a common maximum point closest to EF at k = π/2 while they show different dispersions of about 0.25 and 1 eV, respectively, with a continuous intensity distribution in between. The less dispersive band is almost diminished in another half of the Brillouin zone (k = π/2 to π). These results are consistent with the solution of the one-dimensional Hubbard model as well as the t-J model calculation, suggesting the spin-charge separation in the one-dimensional Cu-O chain. The experimental results are compared with those of the double-chain compound SrCuO2. © 1998 Published by Elsevier Science Ltd.

DOI： 10.1016/S0038-1098(98)00099-4

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier Ltd  

Angle-resolved photoemission spectroscopy has been performed on Sr14-xCaxCu24O41 (x = 0-11.5) which consists of two-leg Cu2O3 ladders and edge-sharing CuO2 chains. A dispersive band with the periodicity of ladder sublattice is observed near the Fermi level (EF) for x=0. The band shows a minimum binding energy at ka= (2n + 1)π/2, suggesting that a Mott-Hubbard gap opens on the ladder. This indicates that copper atoms on the ladder take essentially the divalent state (Cu2+) and a majority of self-doped holes reside on the chain. We found that the Ca substitution causes a transfer of holes from the chain to the ladder. © 1998 Elsevier Science Ltd. All rights reserved.

DOI： 10.1016/S0022-3697(98)00148-6

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier  

We observed a change of Fermi-surface topology of CeSb across the magnetic phase transition using temperature-dependent high-resolution angle-resolved photoemission spectroscopy. The experimental results show that the complicated magnetic structure of CeSb strongly relates to its unique electronic structure as predicted by the anisotropic p-f mixing model. © 1998 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0304-8853(97)00440-X

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier Ltd  

In this paper, we review some of our ARPES results on the superconducting and pseudo gaps in Bi2Sr2CaCu2O8+x. We find that optimally and overdoped samples exhibit a d-wave gap, which closes at the same temperature, Tc, for all k points. In underdoped samples, a leading edge gap is found up to a temperature T* &gt
Tc. We find that T* scales with the maximum low temperature gap, increasing as the doping is reduced. The momentum dependence of the pseudogap is similar to that of the superconducting gap
however, the pseudogap closes at different temperatures for different k points. © 1998 Published by Elsevier Science Ltd. All rights reserved.

DOI： 10.1016/S0022-3697(98)00136-X

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Photoemission spectra of Bi2Sr2CaCu2O8 + δ below Tc show two features near the (π, 0) point of the zone: a sharp peak at low energy and a higher binding energy hump. We find that the sharp peak persists at low energy even as one moves towards (0, 0), while the broad hump shows significant dispersion which correlates well with the normal state dispersion. We argue that these features are naturally explained by the interaction of electrons with a sharp mode which appears only below Tc, and speculate that the latter may be related to the resonance seen in recent neutron data. © 1997 American Physical Society.

DOI： 10.1103/PhysRevLett.79.3506

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We show, by use of angle-resolved photoemission spectroscopy, that underdoped Bi2Sr2CaCu2O8+δ appears to have a large Fermi surface centered at (π, π), even for samples with a Tc as low as 15 K. No clear evidence of a Fermi surface pocket around (π/2, π/2) has been found. These conclusions are based on a determination of the minimum gap locus in the pseudogap regime Tc &lt
T &lt
T*, which is found to coincide with the locus of gapless excitations in momentum space (Fermi surface) determined above T* . These results suggest that the pseudogap is more likely of precursor pairing rather than magnetic origin. © 1997 American Physical Society.

DOI： 10.1103/PhysRevLett.78.2628

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMERICAN PHYSICAL SOC  

DOI： 10.1103/PhysRevLett.78.2272

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A Comment on the Letter by T. Yokoya et al., Phys. Rev. Lett. 76, 3009 (1996). The authors of the Letter offer a Reply. © 1997 The American Physical Society.

DOI： 10.1103/PhysRevLett.78.2271

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier  

We have performed high-resolution angle-resolved photoemission spectroscopy of a non-cuprate layered superconductor Sr2RuO4 to study the electronic structure near the Fermi level (EF). The result shows an extended van Hove singularity (vHs) along the Ru-O bonding direction just like the cuprate, regardless of the character of the electronic states at EF. This suggests that the extended vHs is a general electronic structure of a two dimensional correlated d-electron metal. We also have found that the observed Fermi surface topology (one electron-like Fermi surface sheet centered at Γ point and two hole-like ones centered at X point) is qualitatively different from the calculated Fermi surface topology (two electron-like Fermi sheets centered at Γ point and one hole-like one centered at X point) in the consequence of the existence of the vHs.

DOI： 10.1016/S0921-4534(97)00208-6

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Physical Society of Japan  

The energy band structure of a quasi one-dimensional cuprate SrCuO2 is studied by angle-resolved photoemission spectroscopy and the linear augmented-plane-wave band structure calculation within local density approximation. It was found that the band structure has strongly one-dimensional nature with a finite dispersion along the Cu-O chains. The calculated band structure of SrCuO2 is in good agreement with the angle-resolved photoemission result except for the region near the Fermi level.

DOI： 10.1143/JPSJ.66.1756

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We study the change in the electronic structure of (Formula presented) across the δ-controlled metal-insulator (M-I) transition. X-ray-absorption spectroscopy shows the formation of hole states with substantial (Formula presented) character as a function of δ. Ultraviolet photoemission spectroscopy confirms the gap (Formula presented) closure across the M-I transition due to the doped hole states. The results show that a Mott-Hubbard system can have a charge-transfer character due to the strong hybridization, indicating a very mixed character ground state. © 1997 The American Physical Society.

DOI： 10.1103/PhysRevB.56.12050

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier  

We report angle-resolved photoemission of overdoped Bi1.80Pb0.38Sr2.01CuO6 with different Tc's of 4, 8, and 20 K. All samples show similar large Fermi surfaces, centered at (π, π). However, the spectral function linewidth decreases as doping increases. The 4K sample is found to have the typical Fermi liquid linewidth. Comparison is also made to the two layer Bi2Sr2CaCu2O8 compound.

DOI： 10.1016/S0921-4534(97)00572-8

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Angle-resolved photoemission measurements have been performed on (Formula presented) which consists of two-leg (Formula presented) ladders and edge-sharing (Formula presented) chains. The data are consistent with the existence of a dispersive band near the Fermi level which has the periodicity of the ladder sublattice. The band shows a minimum binding energy at (Formula presented) suggesting that a Mott-Hubbard gap opens on the ladder. The experimental result shows a qualitative agreement with the (Formula presented) ladder calculation. It was also found that replacement of Sr with Ca causes a transfer of holes from the chain to the ladder. © 1997 The American Physical Society.

DOI： 10.1103/PhysRevB.56.7870