Faculty Profiles - YOKOYA Takayoshi

Papers - YOKOYA Takayoshi

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Evidence for a multiple superconducting gap in MgB2 from high-resolution photoemission spectroscopy Reviewed

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

PHYSICAL REVIEW LETTERS 87 ( 17 ) 2001.10

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

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

Changes of electronic structure across the insulator-to-metal transition of quasi-two-dimensional Na-intercalated beta-HfNCl studied by photoemission and x-ray absorption Reviewed

T Yokoya, Y Ishiwata, S Shin, S Shamoto, K Iizawa, T Kajitani, Hase, I, T Takahashi

PHYSICAL REVIEW B 64 ( 15 ) 2001.10

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

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

Determination of the Fermi surface in high-T-c superconductors by angle-resolved photoemission spectroscopy Reviewed

J Mesot, M Randeria, MR Norman, A Kaminski, HM Fretwell, JC Campuzano, H Ding, T Takeuchi, T Sato, T Yokoya, T Takahashi, Chong, I, T Terashima, M Takano, T Mochiku, K Kadowaki

PHYSICAL REVIEW B 63 ( 22 ) 2001.6

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

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

Topology of the fermi surface and band structure near the fermi level in the Ph-doped Bi2Sr2CuO6+delta superconductor Reviewed

T Takeuchi, T Yokoya, S Shin, K Jinno, M Matsuura, T Kondo, H Ikuta, U Mizutani

JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA 114 629 - 634 2001.3

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

Ultra-high resolution photoemission spectroscopy of simple metals: direct observation of superconducting gap and phonon-induced fine structures Reviewed

T Kiss, T Yokoya, A Chainani, S Shin

JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA 114 635 - 639 2001.3

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

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

Ultrahigh-resolution photoemission and resonant-inverse photoemission spectroscopy on CePd3 Reviewed

K Kanai, T Kiss, T Yokoya, G Schmerber, JP Kappler, JC Parlebas, S Shin

JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA 114 741 - 745 2001.3

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

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

Two different types of pseudogaps in high-T-c superconductors Reviewed

T Takahashi, T Sato, T Yokoya, T Kamiyama, Y Naitoh, T Mochiku, K Yamada, Y Endoh, K Kadowaki

JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS 62 ( 1-2 ) 41 - 45 2001.1

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

High-resolution angle-resolved photoemission study of Pb-substituted Bi2201 Reviewed

T Sato, Y Naitoh, T Kamiyama, T Takahashi, T Yokoya, J Mesot, A Kaminski, H Fretwell, JC Campuzano, H Ding, Chong, I, T Terashima, M Takano, K Kadowaki

JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS 62 ( 1-2 ) 157 - 161 2001.1

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

Determination of the Fermi surface in high-Tc superconductors by angle-resolved photoemission spectroscopy Reviewed

J. Mesot, M. Randeria, M. R. Norman, A. Kaminski, H. M. Fretwell, J. C. Campuzano, H. Ding, T. Takeuchi, T. Sato, T. Yokoya, T. Takahashi, I. Chong, T. Terashima, M. Takano, T. Mochiku, K. Kadowaki

Physical Review B - Condensed Matter and Materials Physics 63 ( 22 ) 2245161 - 22451614 2001

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

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

Ultrahigh-resolution photoemission spectroscopy of Ni borocarbides: direct observation of the superconducting gap and a change in gap anisotropy by impurity

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

Physical Review Letters 85 ( 23 ) 4952 - 4955 2000.12

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

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

Hole-state and defect structure of proton conductor SrTiO3 observed by high-resolution X-ray absorption spectroscopy

T. Higuchi, T. Tsukamoto, N. Sata, M. Ishigame, K. Kobayashi, S. Yamaguchi, Y. Ishiwata, T. Yokoya, M. Fujisawa, S. Shin

Solid State Ionics 136-137 261 - 264 2000.11

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

Pseudogap formation in Kondo insulators CeRhSb and CeRhAs studied by ultrahigh-resolution photoemission spectroscopy Reviewed

H Kumigashira, T Sato, T Yokoya, T Takahashi, S Yoshii, M Kasaya

PHYSICA B 281 284 - 285 2000.6

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

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

Angle-resolved photoemission study of 1D chain and two-leg ladder Reviewed

T Sato, T Yokoya, H Fujisawa, T Takahashi, M Uehara, T Nagata, J Akimitsu, S Miyasaka, M Kibune, H Takagi

STRIPES AND RELATED PHENOMENA 263 - 270 2000

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).

Electronic structure in the band gap of lightly doped by high-resolution x-ray absorption spectroscopy

T. Higuchi, T. Tsukamoto, K. Kobayashi, Y. Ishiwata, M. Fujisawa, T. Yokoya, S. Yamaguchi, S. Shin

Physical Review B - Condensed Matter and Materials Physics 61 ( 19 ) 12860 - 12863 2000

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

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

Photoemission spectroscopy of the strong-coupling superconducting transitions in lead and niobium

A. Chainani, T. Yokoya, T. Kiss, S. Shin

Physical Review Letters 85 ( 9 ) 1966 - 1969 2000

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

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

Superconducting gap, pseudogap, and fermi surface of Bi2201: high energy- and momentum-resolution photoemission study

T. Sato, Y. Naitoh, T. Kamiyama, T. Takahashi, T. Yokoya, J. Mesot, A. Kaminski, H. Fretwell, J. C. Campuzano, H. Ding, I. Chong, T. Terashima, M. Takano, K. Kadowaki

Physica C: Superconductivity and its Applications 341-348 2091 - 2094 2000

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

Ti-O Hybridization Effect on Ferroelectric Phase Transition of BaTiO₃(共著)

Higuchi Tohru, Tsukamoto Takeyo, Oka Kunihiko, Yokoya Takayoshi, Tezuka Yasuhisa, Shin Shik

Jpn. J. Appl. Phys. 38 ( 9 ) 5667 - 5669 1999.9

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

Other Link： https://jlc.jst.go.jp/DN/JALC/00063927863?from=CiNii

Spectral evidence for pseudogap formation in kondo insulators CeRhAs and CeRhSb

H. Kumigashira, T. Sato, T. Yokoya, T. Takahashi, S. Yoshii, M. Kasaya

Physical Review Letters 82 ( 9 ) 1943 - 1946 1999.1

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

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

Electronic spectra and their relation to the (π, π) collective mode in high-Tc superconductors

J. C. Campuzano, H. Ding, M. R. Norman, H. M. Fretwell, M. Randeria, A. Kaminski, J. Mesot, T. Takeuchi, T. Sato, T. Yokoya, T. Takahashi, T. Mochiku, K. Kadowaki, P. Guptasarma, D. G. Hinks, Z. Konstantinovic, Z. Z. Li, H. Raffy

Physical Review Letters 83 ( 18 ) 3709 - 3712 1999.1

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

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

Pseudogap of optimally doped La1.85Sr0.15CuO4 observed by ultrahigh-resolution photoemission spectroscopy

T. Sato, T. Yokoya, Y. Naitoh, T. Takahashi, K. Yamada, Y. Endoh

Physical Review Letters 83 ( 11 ) 2254 - 2257 1999.1

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

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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