Faculty Profiles - YOKOYA Takayoshi

Papers - YOKOYA Takayoshi

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24pxA-2 The study of electronic structure of n-type phosphorus-doped diamond by using soft x-ray photoelectron spectroscopy Reviewed

Okazaki H., Yamasaki S., Takano Y., Ishii S., Oguchi T., Yokoya T., Muro T., Nakamura T., Matsushita T., Wakita T., Muraoka Y., Hirai M., Kato S.

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

Angle-resolved photoemission study of the cobalt oxide superconductor NaxCoO2 center dot yH(2)O: Observation of the Fermi surface Reviewed

T. Shimojima, K. Ishizaka, S. Tsuda, T. Kiss, T. Yokoya, A. Chainani, S. Shin, P. Badica, K. Yamada, K. Togano

PHYSICAL REVIEW LETTERS 97 ( 26 ) 2006.12

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

Laser-excited ultrahigh-resolution photoemission spectroscopy of borocarbide superconductor RNi2B2C(R = Y and Er) Reviewed

T. Baba, T. Yokoya, S. Tsuda, T. Kiss, T. Shimojima, S. Shin, T. Togashi, C. T. Chen, C. Q. Zhang, S. Watanabe, T. Watanabe, M. Nohara, H. Takagi

PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 445 46 - 49 2006.10

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

Observation of step-induced magnetic domain formation in La1-xSrxMnO3 thin films by photoelectron emission microscopy Reviewed

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

APPLIED PHYSICS LETTERS 89 ( 11 ) 2006.9

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

Superconducting gap of filled skutterudite superconductor LaRu4P12 studied by sub-meV resolution photoemission spectroscopy Reviewed

S. Tsuda, T. Yokoya, T. Kiss, T. Shimojima, S. Shin, T. Togasi, S. Watanabe, C. Q. Zhang, C. T. Chen, H. Sugawara, H. Sato, H. Harima

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 75 ( 6 ) 2006.6

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

Laser-excited ultrahigh-resolution photoemission study of anisotropic s-wave superconductor YNi2B2C Reviewed

T Baba, T Yokoya, S Tsuda, T Kiss, T Shimojima, S Shin, T Togashi, CT Chen, CQ Zhang, S Watanabe, T Watanabe, M Nohara, H Takagi

PHYSICA B-CONDENSED MATTER 378-80 469 - 470 2006.5

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

Hard X-ray photoelectron emission microscopy as tool for studying buried layers Reviewed

T Wakita, T Taniuchi, K Ono, M Suzuki, N Kawamura, M Takagaki, H Miyagawa, FZ Guo, T Nakamura, T Muro, H Akinaga, T Yokoya, M Oshima, K Kobayashi

JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS 45 ( 3A ) 1886 - 1888 2006.3

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

Bulk- and surface-sensitive high-resolution photoemission study of two mott-hubbard systems: SrVO3 and CaVO3 Reviewed

R Eguchi, T Kiss, S Tsuda, T Shimojima, T Mizokami, T Yokoya, A Chainani, S Shin, IH Inoue, T Togashi, S Watanabe, CQ Zhang, CT Chen, M Arita, K Shimada, H Namatame, M Taniguchi

PHYSICAL REVIEW LETTERS 96 ( 7 ) 2006.2

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

Anisotropic s-wave superconductors studied by angle-resolved photoemission spectroscopy Reviewed

T Yokoya, T Baba, S Tsuda, T Kiss, A Chainani, S Shin, T Watanabe, M Nohara, T Hanaguri, H Takagi, Y Takano, H Kito, J Itoh, H Harima, T Oguchi

JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS 67 ( 1-3 ) 277 - 281 2006.1

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

Laser-excited ultrahigh-resolution photoemission spectroscopy of superconducting Na0.35CoO2 center dot 1.3H(2)O Reviewed

T Shimojima, T Yokoya, T Kiss, A Chainani, S Shin, T Togashi, C Zhang, C Chen, S Watanabe, K Takada, T Sasaki, H Sakurai, E Takayama-Muromachi

JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS 67 ( 1-3 ) 282 - 285 2006.1

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

Experimental investigations on the electronic structure and the low-temperature stability of Cd-based quasicrystals and their 1/1 cubic approximants Reviewed

R Tamura, K Minoda, S Takeuchi, T Takeuchi, K Edagawa, T Kiss, T Yokoya, S Shin

PHILOSOPHICAL MAGAZINE 86 ( 3-5 ) 489 - 497 2006

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

Laser-excited photoemission spectroscopy study of superconducting boron-doped diamond Reviewed

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

SCIENCE AND TECHNOLOGY OF ADVANCED MATERIALS 7 S17 - S21 2006

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

Soft X-ray angle-resolved photoemission spectroscopy of heavily boron-doped superconducting diamond films Reviewed

T. Yokoya, T. Nakamura, T. Matushita, T. Muro, H. Okazaki, M. Arita, K. Shimada, H. Namatame, M. Taniguchi, Y. Takano, M. Nagao, T. Takenouchi, H. Kawarada, T. Oguchi

SCIENCE AND TECHNOLOGY OF ADVANCED MATERIALS 7 S12 - S16 2006

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

Fermi surface topology of borocarbide superconductor YNi2B2C: Angle-resolved photoemission spectroscopy Reviewed

T. Baba, T. Yokoya, S. Tsuda, S. Shin, T. Watanabe, M. Nohara, H. Takagi, T. Oguchi

LOW TEMPERATURE PHYSICS, PTS A AND B 850 683 - + 2006

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).

Temperature-dependent luttinger surfaces Reviewed

T Ito, A Chainani, T Haruna, K Kanai, T Yokoya, S Shin, R Kato

PHYSICAL REVIEW LETTERS 95 ( 24 ) 2005.12

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

Origin of the metallic properties of heavily boron-doped superconducting diamond Reviewed

T Yokoya, T Nakamura, T Matsushita, T Muro, Y Takano, M Nagao, T Takenouchi, H Kawarada, T Oguchi

NATURE 438 ( 7068 ) 647 - 650 2005.12

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

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

K Ishizaka, T Kiss, T Shimojima, T Yokoya, T Togashi, S Watanabe, CQ Zhang, CT Chen, Y Onose, Y Tokura, S Shin

PHYSICAL REVIEW B 72 ( 23 ) 2005.12

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

Evidence for suppressed screening on the surface of high temperature La2-xSrxCuO4 and Nd2-xCexCuO4 superconductors Reviewed

M Taguchi, A Chainani, K Horiba, Y Takata, M Yabashi, K Tamasaku, Y Nishino, D Miwa, T Ishikawa, T Takeuchi, K Yamamoto, M Matsunami, S Shin, T Yokoya, E Ikenaga, K Kobayashi, T Mochiku, K Hirata, J Hori, K Ishii, F Nakamura, T Suzuki

PHYSICAL REVIEW LETTERS 95 ( 17 ) 2005.10

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

Carbon-substitution dependent multiple superconducting gap of Mg B2: A sub-meV resolution photoemission study Reviewed

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

Physical Review B - Condensed Matter and Materials Physics 72 ( 6 ) 2005.8

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

Angle-resolved photoemission study of K0.3MoO3: direct observation of temperature-dependent Fermi surface across the Peierls transition Reviewed

H Ando, T Yokoya, K Ishizaka, S Tsuda, T Kiss, S Shin, T Eguchi, M Nohara, H Takagi

JOURNAL OF PHYSICS-CONDENSED MATTER 17 ( 32 ) 4935 - 4940 2005.8

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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