Faculty Profiles - YOKOYA Takayoshi

Papers - YOKOYA Takayoshi

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Advantage of thin-film filter for reliable photoemission spectroscopy using high-flux discharging lamp Reviewed

T Yokoya, S Tsuda, T Kiss, S Shin, T Mochiku, K Hirata

JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS 43 ( 6A ) 3618 - 3619 2004.6

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

Bulk electronic structure of Na<inf>0.35</inf>CoO<inf>2</inf>·1. 3H<inf>2</inf>O Reviewed

A. Chainani, T. Yokoya, Y. Takata, K. Tamasaku, M. Taguchi, T. Shimojima, N. Kamakura, K. Horiba, S. Tsuda, S. Shin, S. Shin, D. Miwa, Y. Nishino, T. Ishikawa, M. Yabashi, K. Kobayashi, H. Namatame, M. Taniguchi, K. Takada, K. Takada, T. Sasaki, T. Sasaki, H. Sakurai, E. Takayama-Muromachi

Physical Review B - Condensed Matter and Materials Physics 69 ( 18 ) 2004.5

Publishing type：Research paper (scientific journal)

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

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

Bulk electronic structure of Na0.35CoO2 center dot 1.3H(2)O Reviewed

A Chainani, T Yokoya, Y Takata, K Tamasaku, M Taguchi, T Shimojima, N Kamakura, K Horiba, S Tsuda, S Shin, D Miwa, Y Nishino, T Ishikawa, M Yabashi, K Kobayashi, H Namatame, M Taniguchi, K Takada, T Sasaki, H Sakurai, E Takayama-Muromachi

PHYSICAL REVIEW B 69 ( 18 ) 2004.5

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

Bulk electronic structure of Na0.35CoO2 center dot 1.3H(2)O Reviewed

Chainani, A, Yokoya, T, Takata, Y, Tamasaku, K, Taguchi, M, Shimojima, T, Kamakura, N, Horiba, K, Tsuda, S, Shin, S, Miwa, D, Nishino, Y, Ishikawa, T, Yabashi, M, Kobayashi, K, Namatame, H, Taniguchi, M, Takada, K, Sasaki, T, Sakurai, H, Takayama-Muromachi, E

Physical Review B 69 ( 18 ) 180508 2004.5

Experimental evidence for the p-d hybridization in the Cd-Ca quasicrystal: Origin of the pseudogap Reviewed

R Tamura, T Takeuchi, C Aoki, S Takeuchi, T Kiss, T Yokoya, S Shin

PHYSICAL REVIEW LETTERS 92 ( 14 ) 2004.4

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

Contribution of electronic structure to the large thermoelectric power in layered cobalt oxides Reviewed

Tsunehiro Takeuchi, Takeshi Kondo, Tsuyoshi Takami, Hirofumi Takahashi, Hiroshi Ikuta, Uichiro Mizutani, Kazuo Soda, Ryoji Funahashi, Masahiro Shikano, Masashi Mikami, Syunsuke Tsuda, Takayoshi Yokoya, Shik Shin, Takayuki Muro

Physical Review B - Condensed Matter and Materials Physics 69 ( 12 ) 2004.3

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

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

Identical superconducting gap on different Fermi surfaces of Ca(Al0.5Si0.5)(2) with the AlB2 structure Reviewed

S Tsuda, T Yokoya, S Shin, M Imai, Hase, I

PHYSICAL REVIEW B 69 ( 10 ) 2004.3

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

Contribution of electronic structure to the large thermoelectric power in layered cobalt oxides Reviewed

T Takeuchi, T Kondo, T Takami, H Takahashi, H Ikuta, U Mizutani, K Soda, R Funahashi, M Shikano, M Mikami, S Tsuda, T Yokoya, S Shin, T Muro

PHYSICAL REVIEW B 69 ( 12 ) 2004.3

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

Erratum: Electronic structure and superconducting gap of silicon clathrate Ba<inf>8</inf>Si<inf>46</inf> studied with ultrahigh-resolution photoemission spectroscopy (Physical Review B - Condensed Matter and Materials Physics (2001) 64 (172504)) Reviewed

Yokoya, T., Fukushima, A., Kiss, T., Kobayashi, K., Shin, S., Moriguchi, K., Shintani, A., Fukuoka, H., Yamanaka, S.

Physical Review B - Condensed Matter and Materials Physics 70 ( 15 ) 2004

Comparative study of the electronic structure of MgB2 and ZrB2 Reviewed

C. Jariwala, A. Chainani, S. Tsuda, T. Yokoya, S. Shin, Y. Takano, K. Togano, S. Otani, H. Kito

Physical Review B - Condensed Matter and Materials Physics 68 ( 17 ) 2003.11

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

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

Comparative study of the electronic structure of MgB2 and ZrB2 Reviewed

C Jariwala, A Chainani, S Tsuda, T Yokoya, S Shin, Y Takano, K Togano, S Otani, H Kito

PHYSICAL REVIEW B 68 ( 17 ) 2003.11

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

Soft X-ray emission and high-resolution photoemission study of quasi-two-dimensional superconductor NaxHfNCl Reviewed

T Takeuchi, S Tsuda, T Yokoya, T Tsukamoto, S Shin, A Hirai, S Shamoto, T Kajitani

PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 392 127 - 129 2003.10

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

X-ray absorption and resonant photoemission spectroscopy of ZrB2 Reviewed

S Tsuda, R Eguchi, A Kosuge, T Yokoya, A Fukushima, S Shin, A Chainaini, S Otani, Y Takano, K Togano, H Kito

PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 392 259 - 262 2003.10

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

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

Definitive experimental evidence for two-band superconductivity in MgB2 Reviewed

S Tsuda, T Yokoya, Y Takano, H Kito, A Matsushita, F Yin, J Itoh, H Harima, S Shin

PHYSICAL REVIEW LETTERS 91 ( 12 ) 2003.9

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

Soft x-ray energy-dependent angle-resolved photoemission study of CeIrIn5 Reviewed

T Ito, PA Rayjada, N Kamakura, Y Takata, T Yokoya, A Chainani, S Shin, M Nohara, H Takagi

JOURNAL OF PHYSICS-CONDENSED MATTER 15 ( 28 ) S2149 - S2152 2003.7

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

Photoemission results of intermetallic superconductors: Nb3Al and MgB2 Reviewed

T Yokoya, S Tsuda, A Chainani, T Kiss, K Hirata, Y Takano, K Togano, H Kito, H Ihara, S Shin

JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS 63 ( 12 ) 2141 - 2144 2002.12

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

Ultrahigh resolution photoemission study of CeRu4Sb12 Reviewed

K. Kanai, N. Takeda, S. Nozawa, T. Yokoya, M. Ishikawa, S. Shin

J. Phys. Chem. Solids 63 1175 - 1178 2002.12

Photoemission study of pyrochlore superconductor Cd2Re2O7 Reviewed

R. Eguchi, T. Yokoya, T. Baba, M. Hanawa, Z. Hiroi, N. Kamakura, Y. Takata, H. Harima, S. Shin

Physical Review B - Condensed Matter and Materials Physics 66 ( 1 ) 125161 - 125164 2002.7

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

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

Superconducting transitions studied by ultrahigh-resolution photoemission Reviewed

T Yokoya, T Kiss, A Chainani, S Shin

JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA 124 ( 2-3 ) 99 - 105 2002.7

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

Photoemission study of pyrochlore superconductor Cd2Re2O7 Reviewed

R Eguchi, T Yokoya, T Baba, M Hanawa, Z Hiroi, N Kamakura, Y Takata, H Harima, S Shin

PHYSICAL REVIEW B 66 ( 1 ) 2002.7

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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