Faculty Profiles - YOKOYA Takayoshi

Papers - YOKOYA Takayoshi

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Ultraviolet laser photoemission spectroscopy of FeSi: Observation of a opening in density of states Reviewed

K. Ishizaka, T. Kiss, T. Shimojima, T. Yokoya, T. Togashi, S. Watanabe, C. Q. Zhang, C. T. Chen, Y. Onose, Y. Tokura, S. Shin

Physical Review B 72 233202 2008.12

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

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

Near E-F electronic structure of heavily boron-doped superconducting diamond Reviewed

H. Okazaki, T. Yokoya, J. Nakamura, N. Yamada, T. Nakamura, T. Muro, Y. Tamenori, T. Matsushita, Y. Takata, T. Tokushima, S. Shin, Y. Takano, M. Nagao, T. Takenouchi, H. Kawarada, T. Oguchi

JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS 69 ( 12 ) 2978 - 2981 2008.12

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

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

Electronic Structure of Noncentrosymmetric Superconductor Li-2(PdxPt1-x)(3)B Studied by Photoemission Spectroscopy Reviewed

Rikiya Yoshida, Hiroyuki Okazaki, Mitsutoshi Tajima, Takayuki Muro, Izumi Hase, Kozo Okada, Hiroyuki Takeya, Kazuto Hirata, Masaaki Hirai, Yuji Muraoka, Takayoshi Yokoya

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 77 ( 10 ) 2008.10

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

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

Temperature-dependent localized excitations of doped carriers in superconducting diamond Reviewed

K. Ishizaka, R. Eguchi, S. Tsuda, A. Chainani, T. Yokoya, T. Kiss, T. Shimojima, T. Togashi, S. Watanabe, C. -T. Chen, Y. Takano, M. Nagao, I. Sakaguchi, T. Takenouchi, H. Kawarada, S. Shin

PHYSICAL REVIEW LETTERS 100 ( 16 ) 2008.4

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

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

Bulk electronic structure of the antiferromagnetic superconducting phase in ErNi(2)B(2)C Reviewed

T. Baba, T. Yokoya, S. Tsuda, T. Kiss, T. Shimojima, K. Ishizaka, H. Takeya, K. Hirata, T. Watanabe, M. Nohara, H. Takagi, N. Nakai, K. Machida, T. Togashi, S. Watanabe, X. -Y. Wang, C. T. Chen, S. Shin

PHYSICAL REVIEW LETTERS 100 ( 1 ) 2008.1

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

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

Erratum: Bulk screening in core-level photoemission from Mott-Hubbard and charge-transfer systems (Physical Review B - Condensed Matter and Materials Physics (2005) 71, (155102)) Reviewed

M. Taguchi, A. Chainani, N. Kamakura, K. Horiba, Y. Takata, M. Yabashi, K. Tamasaku, Y. Nishino, D. Miwa, T. Ishikawa, S. Shin, E. Ikenaga, T. Yokoya, K. Kobayashi, T. Mochiku, K. Hirata, K. Motoya

Physical Review B - Condensed Matter and Materials Physics 76 ( 16 ) 2007.10

Thickness dependence of magnetic domain formation in La0.6Sr0.4MnO3 epitaxial thin films studied by XMCD-PEEM Reviewed

Toshiyuki Taniuchi, Ryutaro Yasuhara, Hiroshi Kumigashira, Masato Kubota, Hiroyuki Okazaki, Takanori Wakita, Takanori Yokoya, Kanta Ono, Masaharu Oshima, Mikk Lippmaa, Masashi Kawasaki, Hideomi Koinuma

SURFACE SCIENCE 601 ( 20 ) 4690 - 4693 2007.10

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

Bulk screening in core-level photoemission from Mott-Hubbard and charge-transfer systems (vol 71, art no 155102, 2005) Reviewed

M. Taguchi, A. Chainani, N. Kamakura, K. Horiba, Y. Takata, M. Yabashi, K. Tamasaku, Y. Nishino, D. Miwa, T. Ishikawa, S. Shin, E. Ikenaga, T. Yokoya, K. Kobayashi, T. Mochiku, K. Hirata, K. Motoya

PHYSICAL REVIEW B 76 ( 16 ) 2007.10

Magnetic domain structure of a technically patterned ferromagnetic La0.6Sr0.4MnO3 thin film Reviewed

M. Kubota, T. Taniuchi, R. Yasuhara, H. Kumigashira, M. Oshima, K. Ono, H. Okazaki, T. Wakita, T. Yokoya, H. Akinaga, M. Lippmaa, M. Kawasaki, H. Koinuma

APPLIED PHYSICS LETTERS 91 ( 18 ) 2007.10

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

Study of electronic structures for Fe thin films deposited on Si- and C-faces of 4H-SiC substrates by soft X-ray emission spectroscopy Reviewed

M. Hirai, T. Wakita, H. Okazaki, D. Koishihara, Y. Muraoka, T. Yokoya

APPLIED SURFACE SCIENCE 254 ( 1 ) 26 - 28 2007.10

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

Charge-order-maximized momentum-dependent superconductivity Reviewed

T. Kiss, T. Yokoya, A. Chainani, S. Shin, T. Hanaguri, M. Nohara, H. Takagi

NATURE PHYSICS 3 ( 10 ) 720 - 725 2007.10

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

Sub-meV resolution photoemission study on carbon substituted MgB2 Reviewed

S. Tsuda, T. Yokoya, T. Kiss, T. Shimojima, S. Shin, T. Togashi, S. Watanabe, C. Q. Zhang, C. T. Chen, S. Lee, H. Uchiyama, S. Tajima, N. Nakai, K. Machida

PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 460 80 - 83 2007.9

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

Interplay of superconductivity and rattling phenomena in beta-pyrochlore KOs2O6 studied by photoemission spectroscopy Reviewed

T. Shimojima, Y. Shibata, K. Ishizaka, T. Kiss, A. Chainani, T. Yokoya, T. Togashi, X.-Y. Wang, C. T. Chen, S. Watanabe, J. Yamaura, S. Yonezawa, Y. Muraoka, Z. Hiroi, T. Saitoh, S. Shin

PHYSICAL REVIEW LETTERS 99 ( 11 ) 2007.9

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)>= 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

Laser-photoemission study of antiferromagnetic superconductor ErNi2B2C Reviewed

Teruhisa Baba, Takayoshi Yokoya, Shunsuke Tsuda, Takayuki Kiss, Takahiro Shimojima, Kyoko Ishizaka, Shik Shin, Hiroyuki Takeya, Kazuto Hirata, Tadashi Togashi, C. Zhang, C. T. Chen, Shuntaro Watanabe

PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 460 634 - 635 2007.9

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

Recent photoemission studies on MgB2 and related materials Reviewed

Shunsuke Tsuda, Takayoshi Yokoya, Shik Shin

PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 456 ( 1-2 ) 126 - 133 2007.6

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

Core-level electronic structure evolution of heavily boron-doped superconducting diamond studied with hard x-ray photoemission spectroscopy Reviewed

T. Yokoya, E. Ikenaga, M. Kobata, H. Okazaki, K. Kobayashi, A. Takeuchi, A. Awaji, Y. Takano, M. Nagao, I. Sakaguchi, T. Takenouchi, K. Kobayashi, H. Kawarada, T. Oguchi

PHYSICAL REVIEW B 75 ( 20 ) 2007.5

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

Observation of a superconducting gap in boron-doped diamond by laser-excited photoemission spectroscopy Reviewed

K. Ishizaka, R. Eguchi, S. Tsuda, T. Yokoya, A. Chainani, T. Kiss, T. Shimojima, T. Togashi, S. Watanabe, C. -T. Chen, C. Q. Zhang, Y. Takano, M. Nagao, I. Sakaguchi, T. Takenouchi, H. Kawarada, S. Shin

PHYSICAL REVIEW LETTERS 98 ( 4 ) 2007.1

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

Electronic structures of heavily boron-doped superconducting diamond films Reviewed

Takayoshi Yokoya, Hiroyuki Okazaki, Tetsuya Nakamura, Tomohiro Matsushita, Takayuki Muro, Eiji Ikenaga, Masaaki Kobata, Keisuke Kobayashi, Akihisa Takeuchi, Akihiro Awaji, Yoshihiko Takano, Masanori Nagao, Tomohiro Takenouchi, Hiroshi Kawarada, Tamio Oguchi

DIAMOND ELECTRONICS - FUNDAMENTALS TO APPLICATIONS 956 39 - + 2007

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.

Valence band electronic structures of heavily boron-doped superconducting diamond studied by synchrotron photoemission spectroscopy Reviewed

Takayoshi Yokoya, Tetsuya Nakamura, Tomohiro Matsushita, Takayuki Muro, Eiji Ikenaga, Masaaki Kobata, Keisuke Kobayashi, Yoshihiko Takano, Masanori Nagao, Tomohiro Takenouchi, Hiroshi Kawarada, Tamio Oguchi

NEW DIAMOND AND FRONTIER CARBON TECHNOLOGY 17 ( 1 ) 11 - 19 2007

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.

24pXA-1 Soft x-ray photoelectron spectroscopy of heavily B-doped superconducting diamond Reviewed

Okazaki H., Ishii S., Iriyama S., Kawarada H., Yokoya T., Muro T., Nakamura T., Matsushita T., Wakita T., Muraoka Y., Hirai M., Takano Y.

Meeting Abstracts of the Physical Society of Japan 62 ( 0 ) 665 - 665 2007

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