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

Abstract

We report the direct evidence of valence fluctuation in the thermoelectric material Yb₃Si₅ by using synchrotron X-ray photoemission spectroscopy. The Yb 3d core level photoemission spectrum of a Yb₃Si₅ single crystal shows clear separation of the Yb²⁺ peak and Yb³⁺ multiplet peaks. From the spectral weight of each component, the mean valence of Yb ions is estimated to be +2.67 at 20 K. Furthermore, a Kondo resonance peak just below the Fermi energy is also observed by high-resolution soft X-ray photoemission spectroscopy, suggesting the valence fluctuating state in Yb₃Si₅.

DOI： 10.35848/1347-4065/aca911

Other Link： https://iopscience.iop.org/article/10.35848/1347-4065/aca911/pdf

Publishing type：Research paper (scientific journal)  

DOI： 10.35848/1347-4065/accfd7

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

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

In this study, we report a large magneto-thermal conductivity effect, potentially usable in heat flow switches and thermoelectric devices, in Ag2Te over a wide temperature range, including room temperature. When a magnetic field of mu H-0 = 9 T is applied to Ag2Te at 300 K along the direction perpendicular to the heat and electric currents, the thermal conductivity kappa decreases by a remarkable 61%. This effect is mainly caused by the suppressed electronic thermal conductivity in association with a significant magnetoresistance effect, but the suppression of the thermal conductivity is larger than that of the electrical conductivity, presumably due to a field-induced decrease in the Lorenz ratio. Its very low lattice thermal conductivity, as low as 0.5 W m(-1) K-1, also greatly contributes to the large relative magneto-thermal conductivity effect. The significant decrease in thermal conductivity and the 18% increase in the Seebeck coefficient S lead to a nearly 100% increase in the thermoelectric figure of merit zT = S-2 sigma T kappa(-1) despite the 43% decrease in electrical conductivity sigma.

DOI： 10.1063/5.0131326

Publishing type：Research paper (international conference proceedings)  

Publishing type：Research paper (scientific journal)  

DOI： 10.1063/5.0095987

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

Publishing type：Research paper (scientific journal)  

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

Publishing type：Research paper (scientific journal)   Publisher：AIP publishing  

Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing LLC  

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

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

DOI： 10.1063/5.0073038

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

DOI： 10.35848/1347-4065/ac3723

Other Link： https://iopscience.iop.org/article/10.35848/1347-4065/ac3723/pdf

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

DOI： 10.1088/1361-6463/abcfe7

Other Link： https://iopscience.iop.org/article/10.1088/1361-6463/abcfe7/pdf

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

Publishing type：Research paper (scientific journal)  

In this study, the strategies of modifying the electronic structure, tuning carrier concentration, and nanostructuring were implemented to improve the power factor and reduce the thermal conductivity of Si-Ge simultaneously. The Si0.65-xGe0.32Ni0.03Bx (x = 0.01, 0.02, 0.03, and 0.04) nanostructure was synthesized by high-energy ball-milling. Subsequently, a high-pressure with low-temperature sintering process was carried out. A small amount of nickel (Ni) and boron (B) was introduced into Si-Ge to modify the electronic structure and optimize the carrier concentration. The sintered sample Si0.62Ge0.32Ni0.03B0.03 showed a high power factor of 2.3 mW m-1 K-2 at 1000 K, which was influenced by the modified electronic structure and optimized carrier concentration. In addition, the thermal conductivity was effectively decreased to 1.47 W m-1 K-1 due to the phonon scattering by the micro to nanoscale grain boundaries. Ultimately, a large dimensionless figure of merit ZT ∼1.56 was obtained at 1000 K. Therefore, following these strategies can improve the thermoelectric performance of earth-abundant, environmentally friendly, and nontoxic Si-Ge material.

DOI： 10.1021/acsaelm.1c01075

Publishing type：Research paper (scientific journal)   Publisher：The Japan Institute of Metals and Materials  

Publishing type：Research paper (scientific journal)   Publisher：Japan Institute of Metals  

DOI： 10.2320/matertrans.e-m2020847

Publishing type：Research paper (international conference proceedings)   Publisher：The Japan Institute of Metals and Materials  

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

Publishing type：Research paper (scientific journal)  

We investigated the 5d element substitution effect on the thermal conductivity in Fe2VAl-based superlattice thin films epitaxially grown on a MgO (100) substrate. We found unique crossover behavior in the period dependences of thermal conductivity for Ta-free Fe2VAl and Fe2(V,Ta)Al-based superlattices. The Fe2(V,Ta)Al-based superlattices with periods more than 40 nm appeared to have much lower thermal conductivity than Ta-free Fe2VAl-based superlattices due to the substitution of V by Ta. Unexpectedly, at a shorter periodic length, less than 20 nm, Fe2(V,Ta)Al-based superlattices appeared to have higher thermal conductivity than Ta-free Fe2VAl-based superlattices despite such a heavy element substitution by Ta. This surprising experimental fact was well accounted for with theoretical calculations, which predicted the dominant contribution of phonons having a shorter mean free path in the Ta substituted samples.

DOI： 10.1103/PhysRevB.102.104301

Publishing type：Research paper (scientific journal)  

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

To understand the colossal Seebeck effect recently found in metallic Cu2Se at the phase transition from low-to high-temperature phase, we investigated the behaviors of thermoelectric properties during the phase transition of Cu2S and Ag2Se, both of which possess a phase transition in the same manner as Cu2Se. It has clearly revealed that the colossal Seebeck effect reported for metallic Cu2Se was not observable in Ag2Se but in Cu2S. The main reason for this difference between Cu2S and Ag2Se dwells in the difference of the phase diagrams: presence and absence of two-phase region where the low- and high-temperature phases coexist together in a stable condition. Unique temperature gradient, that allows us to spatially separate the two phases, produced a dynamical variation of carrier concentration in the low-temperature phase and, hence, the colossal Seebeck effect during the phase transition. (C) 2020 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jallcom.2020.154155

Publishing type：Research paper (scientific journal)   Publisher：Springer US  

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

© 2019, The Minerals, Metals & Materials Society. In the present work, Si65-xGe31Au4Bx (x = 1, 2, 3, and 4) nanocomposites were synthesized by ball milling and spark plasma sintering to investigate the boron concentration dependence of thermoelectric properties. The electrical resistivity observed for samples made at 2 ≤ x ≤ 4 was almost constant over a wide temperature from 300 K to 1000 K, indicating the strongest scattering limit with disordering. All samples possessed the large Seebeck coefficient, mainly, B-1% and B-3% doped sample shows the significant Seebeck coefficient exceeding 450 μVK−1, which was presumably attributed to a sharp peak in the electronic density of states near the valence band edge. As a result, the largest power factor of 2.1 mWm−1K−2 was observed at x = 3. In addition, the lowest thermal conductivity less than 1.3 Wm−1K−1 was observed for x = 2,3, and 4. The significant power factor, together with low thermal conductivity naturally led to obtain high ZT value of 1.63 at 973 K. This value is much higher than that of previously reported other Si-Ge based nanocomposites.

DOI： 10.1007/s11664-019-07857-5

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

© 2019, The Minerals, Metals & Materials Society. Materials best-suited for direct application exhibit a high thermoelectric figure of merit (ZT) close to unity, from room temperature to ∼ 400 K. In this work, we investigated the thermoelectric behavior of Ag2Se, and a sulfur substituted Ag2Se system, i.e. Ag2S0.2Se0.8 and Ag2S0.4Se0.6 , in the temperature range of 300 K to 500 K. With strong anharmonic lattice vibration and semiconducting electronic structure, these materials showed thermal conductivity less than 1 W m−1 K−1, electrical resistivity ∼ 1 mΩ cm, together with moderate Seebeck coefficient values of ∼ − 140 μV K−1 in the low-temperature phase. We were able to achieve ZT equal to unity in a wide temperature range of 350–400 K, with a maximum value of ZT = 1.08 at 350 K for the Ag2S0.4Se0.6 material.

DOI： 10.1007/s11664-019-07879-z

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

© 2020, The Minerals, Metals & Materials Society. Thermal diodes, in which the magnitude of the heat flow |J| changes with its direction, are a key technology for constructing heat management systems. Composite thermal diodes are composed of two materials possessing opposite temperature dependence of thermal conductivity. Silver chalcogenides Ag2(S,Se,Te) were employed in this work because of their drastic change in thermal conductivity, with a phase transition in the temperature range of 300 K < T < 450 K. A jump of 200% to 500% in their thermal conductivity is observed at the phase-transition temperature, most likely due to the change in electron thermal conductivity. A thermal diode consisting of Ag2S0.6Se0.4 and Ag2S0.1Te0.9, both of which were selected by considering the measured thermal conductivity, has been prepared, and its performance evaluated. The developed thermal diode was confirmed to possess a thermal rectification ratio (TRR) = |Jlarge|/|Jsmall| = 2.7 ± 0.1 when installed between two heat reservoirs kept at TH = 413 K and TL = 300 K. Notably, this TRR is the highest ever reported for a solid-state thermal diode.

DOI： 10.1007/s11664-020-07964-8

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

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

© 2020 American Physical Society. The thermal transport properties of M/Fe2(V,Ta)Al/M (M=Mo, W) multilayer thin films, which were epitaxially grown on the [100] MgO single-crystal substrates by means of a radio-frequency magnetron sputtering system, were systematically investigated using a picosecond pulsed laser heating time-domain thermoreflectance method. Not only the thermal diffusivity but also the boundary thermal resistance was strongly affected by both the Ta substitution for V in Fe2VAl and the elements in the metallic layer of top and bottom sides. From the first-principles density-functional theory calculations, we found that such changes in heat transport properties were directly related to the intrinsic phonon dispersion. In the Fe2(V,Ta)Al layer thickness region of less than 14 nm, the boundary thermal resistances of all thin-film series abruptly decreased with decreasing the thickness, which was due to the coherently ordered interfacial structure.

DOI： 10.1103/PhysRevB.101.104312

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

Publishing type：Research paper (scientific journal)  

© 2020 Author(s). We have investigated the electronic structure of a potential thermoelectric material, higher manganese silicide, and its solid solutions with supersaturated Re by means of hard x-ray photoemission spectroscopy. The unusual Re substitution dependence of thermoelectric properties in the higher manganese silicide, reducing the electrical resistivity without altering the Seebeck coefficient, can be explained in terms of the variation of the density of states at the Fermi level and its energy derivative. Such an electronic structure variation contributes to an improvement in the thermoelectric performance in combination with the suppressed thermal conductivity.

DOI： 10.1063/1.5128185

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

Authorship：Lead author   Publishing type：Research paper (scientific journal)   Publisher：AIP  

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

Publishing type：Research paper (scientific journal)  

© 2019 The Japan Society of Applied Physics. We report the thermoelectric properties of valence fluctuating material Yb5Si3. The temperature dependent Seebeck coefficient of Yb5Si3 shows a negative peak at 10 K and positive broad peak around 100 K together with two sign changes. Such a behavior is different from that of typical heavy fermion systems. We demonstrate that the thermoelectric properties of Yb5Si3 including the electrical resistivity are most likely caused by the heterogeneous Yb valence states reflecting the crystallographically inequivalent Yb sites.

DOI： 10.7567/1347-4065/ab5b85

Publishing type：Research paper (scientific journal)   Publisher：Japan Institute of Metals  

DOI： 10.2320/matertrans.e-m2020844

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

© 2020, The Minerals, Metals & Materials Society. Determination of thermoelectric properties is rather a challenging task, especially during a phase transition, where a drastic change in magnitude of thermoelectric properties is generally observed. In this study, we performed a precise measurement of the Seebeck coefficient on Cu2Se, which was recently reported to possess a colossal Seebeck coefficient with metallic electrical conductivity during a structural phase transition. We confirmed that the colossal Seebeck effect was stable over a long period of time, as high as −0.9 mV under an applied temperature gradient of 1 K. This fact clearly proved that the colossal Seebeck effect observable in metallic Cu2Se is potentially usable for a variety of thermoelectric applications.

DOI： 10.1007/s11664-019-07884-2

Publishing type：Research paper (scientific journal)   Publisher：Japanese Society of Applied Physics  

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

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

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

The enhanced value of the dimensionless figure of merit was achieved in a P-type Si-Ge alloy containing a small amount of Au and B, Si0.62Ge0.31B0.03Au0.04. After the 6;h ball milling, the sample was sintered at 873;K under 400;MPa with a soaking time of 4;h. The precipitation of (Au) occurred in Si-Ge alloy matrix. The large Seebeck coefficient of 464 ?VK?1 was achieved at 973;K, which might be due to the sharp peak, that consists mainly of 5d states of Au, near the top of the valence band in the electronic density of states. Assisted by the values of electrical resistivity ?10 m?cm and thermal conductivity 1.5 Wm(?1);K-?1, the large Seebeck coefficient led to a value of ZT exceeding 1.6, which is much higher than that of previously reported Si-Ge-based composites.

DOI： 10.7567/1347-4065/ab4fb9

Publishing type：Research paper (scientific journal)   Publisher：日本熱電学会  

Publishing type：Research paper (scientific journal)   Publisher：日本熱電学会  

Publishing type：Research paper (scientific journal)   Publisher：Japanese Society of Applied Physics  

Publishing type：Research paper (scientific journal)   Publisher：日本熱電学会  

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

A comprehensive study of thermoelectric power on Y/Pb-doped Bi1.7Pb0.4Sr2Ca1-xYxCuO8+delta (Bi2212) single crystals with a varying doping level was carried out. By proposing new definitions, the measurements provided us with a new phase diagram of a typical representative of cuprate superconductors. The diagram disclosed novel phase regions found out deep in the pseudogap state, where an ordering such as charge/spin density has been observed. We propose that the results are connected to two competing phenomena depending on the doping content. The origin near the optimal doping is suggested to reflect both new order formation associated with a Fermi surface reconstruction and preformation of Cooper pairs, whereas towards underdoped region a competing order overcomes the superconductivity leading to the well-known dome of superconductivity. A dip of superconducting transition with charge density wave dome and polaron interpretation to the acquired data are additionally discussed. By this approach, we have proved that electronic structure investigations via thermoelectric power studies lead to the novel results with a significant agreement with the previous studies.

DOI： 10.7566/JPSJ.88.064711

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

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

High-performance Si-Ge-based thermoelectric materials were prepared simultaneously using nano-structuring and electronic structure modifications. Density functional theory calculations predicted that Fe atoms in the Si-Ge alloy would constructively modify the electronic structure to significantly increase the Seebeck coefficient, experimentally confirmed as vertical bar S vertical bar > 517 +/- 20 mu VK-1 at 673 K. Dense bulk samples made of nano-grains possessed very small thermal conductivity, kappa < 0.80 +/- 0.10 W m(-1) K-1 at T < 873 K. Very large ZT values exceeding 1.00 were obtained in the temperature range 710 K < T < 873 K, with a maximal value of 1.88 at 873 K. (C) 2019 The Japan Society of Applied Physics

DOI： 10.7567/1882-0786/ab08b7

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

© 2018 Elsevier B.V. We investigated the electronic structure of a thermoelectric material, Heusler-type Fe2VAl thin film, by hard X-ray photoemission spectroscopy and electronic band structure calculation to understand the improvement process of the thermoelectric properties of this material. Electronic structure of the Fe2VAl thin film showed a semi-metallic electronic structure with a pseudo-gap at the Fermi level. The observed electronic structure of the Fe2VAl thin film suggests that an optimal carrier doping exists to best control the n- and p-type thermoelectric properties and enhance the power factors.

DOI： 10.1016/j.elspec.2018.12.003

Publishing type：Research paper (scientific journal)   Publisher：Nature Publishing Group  

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

Both electrical conductivity sigma and Seebeck coefficient S are functions of carrier concentration being correlated with each other, and the value of power factor S-2 sigma is generally limited to less than 0.01 W m(-1) K-2. Here we report that, under the temperature gradient applied simultaneously to both parallel and perpendicular directions of measurement, a metallic copper selenide, Cu2Se, shows two sign reversals and colossal values of S exceeding +/- 2 mV K-1 in a narrow temperature range, 340 K < T < 400 K, where a structure phase transition takes place. The metallic behavior of sigma possessing larger magnitude exceeding 600 S cm(-1) leads to a colossal value of S-2 sigma = 2.3 W m(-1) K-2. The small thermal conductivity less than 2 W m(-1) K-1 results in a huge dimensionless figure of merit exceeding 400. This unusual behavior is brought about by the self-tuning carrier concentration effect in the low-temperature phase assisted by the high-temperature phase.

DOI： 10.1038/s41467-018-07877-5

Publishing type：Research paper (scientific journal)   Publisher：Japanese Society of Applied Physics  

Publishing type：Research paper (scientific journal)   Publisher：The Japan Institute of Metals and Materials  

Publishing type：Research paper (scientific journal)  

© 2018 American Physical Society. The evolution of Fermi surface (FS) states of NdFeAs1-xPxO0.9F0.1 single crystals with As/P substitution has been investigated. The critical temperature Tc and the power-law exponent (n) of temperature-dependent resistivity [ρ(T)=ρ0+ATn] show a clear correlation above x=0.2, suggesting that Tc is enhanced with increasing bosonic fluctuations in the same type of FS state. Around x=0.2, all the transport properties show anomalies, indicating that x∼0.2 is the critical composition of a drastic FS change. Angle-resolved photoemission spectroscopy has more directly revealed the distinct change of FS around x=0.2, that one hole FS disappears at the Brillouin zone center and the other FS with a propellerlike shape appears at the zone corner with decreasing x. These results are indicative of the existence of two types of FS states with different nesting conditions that are related with two Tc-raising mechanisms in this system.

DOI： 10.1103/PhysRevB.98.100501

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

Copyright © 2018 The Authors. A valence critical end point existing near the absolute zero provides a unique case for the study of a quantum version of the strong density fluctuation at the Widom line in the supercritical fluids. Although singular charge and orbital dynamics are suggested theoretically to alter the electronic structure significantly, breaking down the standard quasi-particle picture, this has never been confirmed experimentally to date. We provide the first empirical evidence that the proximity to quantum valence criticality leads to a clear breakdown of Fermi liquid behavior. Our detailed study of the mixed valence compound a-YbAlB4 reveals that a small chemical substitution induces a sharp valence crossover, accompanied by a pronounced non-Fermi liquid behavior characterized by a divergent effective mass and unusual T/B scaling in the magnetization.

DOI： 10.1126/sciadv.aao3547

Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN INST METALS  

The thermoelectric properties of the Fe(V0.955-xHf0.045Tix)Sb half-Heusler (HH) phase were studied in the temperature range from 300 to 800 K. We employed Hf- and Ti-substitutions for V in the FeVSb-based HH phase for reducing the lattice thermal conductivity and tuning the carrier concentration, respectively. The samples prepared with the composition of Fe(V(0.955-x)Hf0.045Ti(x))Sb (0.000 <= x <= 0.075) consisted almost solely of the half-Heusler phase. We succeeded in significantly decreasing the lattice thermal conductivity kappa(lat) approximate to 2.1 W m(-1)K(-1) for Fe(V0.88Hf0.045Ti0.075)Sb while keeping appropriate carrier concentration for the optimized transport properties. However, the highest ZT value was not significantly increased but kept rather low at 0.55, mainly due to the reduction in the Seebeck coefficient. The mechanism leading to the reduction in Seebeck coefficient was revealed by the means of first principles band calculations.

DOI： 10.2320/matertrans.M2018171

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

© 2017 American Physical Society. We study the electronic structure of Tsai-type cluster-based quasicrystalline approximants, Au64Ge22Yb14 (AGY-I), Au63.5Ge20.5Yb16 (AGY-II), and Zn85.4Yb14.6 (Zn-Yb), by means of photoemission spectroscopy. In the valence band hard x-ray photoemission spectra of AGY-II and Zn-Yb, we separately observe a fully occupied Yb 4f state and a valence fluctuation derived Kondo resonance peak, reflecting two inequivalent Yb sites, a single Yb atom in the cluster center and its surrounding Yb icosahedron, respectively. The fully occupied 4f signal is absent in AGY-I containing no Yb atom in the cluster center. The results provide direct evidence for a heterogeneous valence state in AGY-II and Zn-Yb.

DOI： 10.1103/PhysRevB.96.241102

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

The thermoelectric properties of the FeV1-xTixSb half-Heusler (HH) phase were studied in the temperature range from 300 to 700 K. All the samples prepared with the compositions of FeV1-xTixSb (0 <= x <= 0.2) possessed a positive Seebeck coefficient, while the sample with x = 0 showed a negative sign. FeV0.80Ti0.20Sb exhibited a large positive Seebeck coefficient exceeding 300 mu VK-1 and a low electrical resistivity below 2.00 +/- 0.20 m Omega cm at 700 K, leading to a large power factor PF = 4.40 +/- 0.50 x 10(-3)W m(-1) K-2. The thermal conductivity of FeV1-xTixSb (x > 0) at 300K decreased to less than half of that of the sample at x = 0, probably because the disordering caused by the Ti substitution enhanced the scattering probability of phonons. The consistency between measured and calculated properties indicates that FeV1-xTixSb HH is suitable as a new practical thermoelectric material. (C) 2017 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.56.111202

Publishing type：Research paper (scientific journal)  

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

The heavily underdoped (Bi,Pb)(2)Sr-2(Ca,Y)Cu2O8+delta single crystals were successfully grown by conventional floating zone technique with a very slow growth rate of 0.20 mm/h. The superconducting gap of prepared single crystals was investigated by means of laser induced ARPES measurement. We found that the superconducting gap was kept almost unchanged from optimally doped region, T-c = 92 K, to heavily underdoped region, T-c similar to 12 K. The result suggest that the pair-formation-energy in underdoped region is similar to that in optimally doped region, and the superconducting temperature is presumably suppressed by the pseudogap which increase with decreasing hole concentration. (C) 2017 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.elspec.2017.01.007

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

Since the discovery of unconventional quantum criticality in the Au-Al-Yb quasicrystal and pressure-induced quantum criticality in its approximant, quasicrystals and approximants with an icosahedral cluster of atoms have attracted much attention. Here we report magnetic, thermodynamic, transport, and hard x-ray photoemission spectroscopy experiments on the Ce-based approximant Ag-In-Ce. These results indicate the heavy fermion behavior of the 4f electrons and provide convincing evidence of a magnetic phase transition into a spin-glass-like short-rangeordered state at a low temperature. The Ag-In-Ce alloy is the first Ce-based heavy fermion approximant to a Tsai-type quasicrystal. Reflecting its unique crystal structure, the approximant shows an electrical resistivity distinct from that of traditional Kondo lattices.

DOI： 10.7566/JPSJ.86.093702

Publishing type：Research paper (scientific journal)  

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

The dynamic interaction between the traveling charges and the molecular vibrations is critical for the charge transport in organic semiconductors. However, a direct evidence of the expected impact of the charge-phonon coupling on the band dispersion of organic semiconductors is yet to be provided. Here, we report on the electronic properties of rubrene single crystal as investigated by angle resolved ultraviolet photoelectron spectroscopy. A gap opening and kink-like features in the rubrene electronic band dispersion are observed. In particular, the latter results in a large enhancement of the hole effective mass (&gt;1.4), well above the limit of the theoretical estimations. The results are consistent with the expected modifications of the band structures in organic semiconductors as introduced by hole-phonon coupling effects and represent an important experimental step toward the understanding of the charge localization phenomena in organic materials.

DOI： 10.1038/s41467-017-00241-z

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

Inducing magnetism into topological insulators is intriguing for utilizing exotic phenomena such as the quantum anomalous Hall effect (QAHE) for technological applications. While most studies have focused on doping magnetic impurities to open a gap at the surface-state Dirac point, many undesirable effects have been reported to appear in some cases that makes it difficult to determine whether the gap opening is due to the time-reversal symmetry breaking or not. Furthermore, the realization of the QAHE has been limited to low temperatures. Here we have succeeded in generating a massive Dirac cone in a MnBi2Se4/Bi2Se3 heterostructure, which was fabricated by self-assembling a MnBi2Se4 layer on top of the Bi2Se3 surface as a result of the codeposition of Mn and Se. Our experimental results, supported by relativistic ab initio calculations, demonstrate that the fabricated MnBi2Se4/Bi2Se3 heterostructure shows ferromagnetism up to room temperature and a clear Dirac cone gap opening of similar to 100 meV without any other significant changes in the rest of the band structure. It can be considered as a result of the direct interaction of the surface Dirac cone and the magnetic layer rather than a magnetic proximity effect. This spontaneously formed self-assembled heterostructure with a massive Dirac spectrum, characterized by a nontrivial Chern number C = -1, has a potential to realize the QAHE at significantly higher temperatures than reported up to now and can serve as a platform for developing future topotronics devices.

DOI： 10.1021/acs.nanolett.7b00560

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

The electronic structures of the highest occupied molecular orbital (HOMO) or the HOMO-derived valence bands dominate the transport nature of positive charge carriers (holes) in organic semiconductors. In the present study, the valence-band structures of single-crystal pentacene and the temperature dependence of their energy momentum dispersion relations are successfully demonstrated using angle-resolved ultraviolet photoelectron spectroscopy(ARUPS). For the shallowest valence band, the intermolecular transfer integral and effective mass of the holes are evaluated as 43.1 meV and 3.43 times the electron rest mass, respectively, at room temperature along the crystallographic direction for which the widest energy dispersion is expected. The temperature dependence of the ARUPS results reveals that the transfer integral values (hole effective mass) are enhanced (reduced) by similar to 20% on cooling the sample to 110 K.

DOI： 10.1021/acs.jpclett.7b00082

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

Publishing type：Research paper (scientific journal)  

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

Carrier concentration dependence of the superconducting gap observable around the nodal region was investigated in detail for a series of high-Tc cuprate superconductors, Bi-2(Sr,La)(2)CuO6+delta, by means of ultra-high-resolution laser-induced angle resolved photoemission spectroscopy. We found that the gap can be expressed by the simple d-wave gap form, Delta = Delta(0) cos(2 theta), at least around the node even in the heavily underdoped samples. The gap size Delta(0) increases with decreasing hole concentration in the overdoped region, and starts to decrease after taking a plateau that extends over a relatively wide hole concentration, p, ranging 0.16 <= p <= 0.26. The critical temperature of superconductivity, T-c, decreases more drastically than Delta(0) with decreasing hole concentration. This fact indicates that T-c is not simply determined by Delta(0), and we argue that the reduction of Tc with decreasing hole concentration in the underdoped regions is caused by a synergy effect of the decreasing Delta(0) and the development of pseudogap.

DOI： 10.7566/JPSJ.85.104710

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

Time-resolved hard X-ray photoelectron spectroscopy (trHAXPES) using microfocused X-ray free-electron laser (XFEL, h nu = 8 keV) pulses as a probe and infrared laser pulses (h nu = 1.55 eV) as a pump is employed to determine intrinsic charge-carrier recombination dynamics in La:SrTiO3. By means of a combination of experiments and numerical N-body simulations, we first develop a simple approach to characterize and decrease XFEL-induced vacuum space-charge effects, which otherwise pose a serious limitation to spectroscopy experiments. We then show that, using an analytical mean-field model, vacuum space-charge effects can be counteracted by pump laser-induced photoholes at high excitation densities. This provides us a method to separate vacuum space-charge effects from the intrinsic charge-carrier recombination dynamics in the time domain. Our trHAXPES results thus open a route to studies of intrinsic charge-carrier dynamics on picosecond time scales with lateral spatial resolution on the micrometer scale.

DOI： 10.1038/srep35087

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

We have investigated the effect of heavy-element (W) substitution on the thermoelectric properties of higher manganese silicide (HMS). Samples were prepared by arc melting followed by liquid quenching, where the latter assisted in achieving higher solubility for tungsten. We observed that Mn34.6W1.8Si63.6 was a p-type material, whereas simultaneous substitution of 12 at.% Fe made the higher manganese silicide an n-type material. The optimal carrier concentration was obtained by simultaneous substitution of Fe and W for Mn atoms. Although the samples were metastable, we successfully obtained bulk samples by a low-temperature (970 K), high-pressure (> 100 MPa), long-duration sintering process. The lattice thermal conductivity was effectively reduced by W substitution,

DOI： 10.1007/s11664-016-4688-x

Publishing type：Research paper (scientific journal)  

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

A synergistic effect between strong electron correlation and spin-orbit interaction has been theoretically predicted to realize new topological states of quantum matter on Kondo insulators (KIs), so-called topological Kondo insulators (TKIs). One TKI candidate has been experimentally observed on the KI SmB6(001), and the origin of the surface states (SS) and the topological order of SmB6 has been actively discussed. Here, we show a metallic SS on the clean surface of another TKI candidate YbB12(001) using angle-resolved photoelectron spectroscopy. The SS shows temperature-dependent reconstruction corresponding to the Kondo effect observed for bulk states. Despite the low-temperature insulating bulk, the reconstructed SS with c-f hybridization is metallic, forming a closed Fermi contour surrounding (Gamma) over bar on the surface Brillouin zone and agreeing with the theoretically expected behaviour for SS on TKIs. These results demonstrate the temperature-dependent holistic reconstruction of two-dimensional states localized on KIs surface driven by the Kondo effect.

DOI： 10.1038/ncomms12690

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

We present an angle-resolved photoemission study of the electronic structure of the three-dimensional pyrochlore iridate Nd_{2}Ir_{2}O_{7} through its magnetic metal-insulator transition. Our data reveal that metallic Nd_{2}Ir_{2}O_{7} has a quadratic band, touching the Fermi level at the Γ point, similar to that of Pr_{2}Ir_{2}O_{7}. The Fermi node state is, therefore, a common feature of the metallic phase of the pyrochlore iridates. Upon cooling below the transition temperature, this compound exhibits a gap opening with an energy shift of quasiparticle peaks like a band gap insulator. The quasiparticle peaks are strongly suppressed, however, with further decrease of temperature, and eventually vanish at the lowest temperature, leaving a nondispersive flat band lacking long-lived electrons. We thereby identify a remarkable crossover from Slater to Mott insulators with decreasing temperature. These observations explain the puzzling absence of Weyl points in this material, despite its proximity to the zero temperature metal-insulator transition.

DOI： 10.1103/PhysRevLett.117.056403

Authorship：Last author   Publishing type：Research paper (scientific journal)  

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

We report a comparative study of the electronic structure of the compounds LaTe and CeTe, both of which crystallize in the rock salt structure. LaTe is a paramagnetic metal while CeTe is known to exhibit anomalous Kondo-like transport behaviour and undergoes a transition to a complex magnetically ordered state at low temperature (T-N = 2.2 K). We carry out hard X-ray photoelectron spectroscopy (HAXPES) of the core-levels and valence band of LaTe and CeTe at T = 20 K, in order to characterize their intrinsic electronic structure, and to address the role of Kondo effect on the electronic structure of CeTe. The bulk sensitive core level HAXPES spectra show evidence of screened features in the La 3d and Ce 3d states mixed with plasmon features. From a careful analysis of the Te, La and Ce derived core levels, we separate out the respective origins of the satellites and show that CeTe indeed exhibits definitive but weak f(0) and f(2) satellites due to Kondo screening, in addition to the main f(1) peak. The comparison of the valence band spectra of CeTe obtained using HAXPES and soft X-ray PES clearly identifies the Ce 4f(1) derived features. Resonant photoelectron spectrosocopy across the Ce 3d - 4f threshold confirms the Ce 4f(1) final state at the Fermi level, corresponding to the tail of the Kondo resonance feature which occurs above the Fermi level, while the Ce 4f(0) final state feature is observed at a binding energy of 2.4 eV. The 4f(0) and 4f(1) final states show giant resonances compared to the off-resonant spectra. However, in contrast to typical Kondo systems, the tail of the Ce 4f(1) Kondo resonance at the Fermi level is relatively suppressed compared to the Ce 4f(0) feature, which exhibits an unusually strong resonance enhancement. The results are indicative of a weakened Kondo effect which favours the magnetically ordered ground state in CeTe. (C) 2015 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.elspec.2015.11.013

Publishing type：Research paper (scientific journal)  

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

In this study, we developed a higher manganese silicide (HMS) that possesses a high dimensionless figure of merit ZT exceeding unity. HMSs containing a larger amount of Re than its solubility limit were prepared by the liquid quenching technique, and the obtained metastable HMSs showed good thermal stability to enable pulse current sintering at 1240 K. The lattice thermal conductivity was effectively reduced with increasing Re concentration, whereas the electron transport properties were not greatly affected. Consequently, the ZT of p-type HMS increased to 1.04 at 6 at.% Re from 0.4 of the Re-free sample. (C) 2016 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.55.020301

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

Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Applied Physics  

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

We report on the electronic structure of the 111-type iron pnictide superconductor LiFeAs as a function of temperature using angle-resolved photoemission spectroscopy. Below approximately 50 K, both the dyz hole band at the Z point and the dxz/yz electron band at the A point shift to a higher binding energy side. However, at the high-symmetry points Z,A,Γ, and M, the remaining bands are almost independent of temperature. One of the possible scenarios for these observations is that a strong, three-dimensional orbital-dependent correlation exists in the normal state of LiFeAs in relation to short-range spin fluctuations.

DOI： 10.1103/PhysRevB.93.024503

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

We report on the electronic structure of the 111-type iron pnictide superconductor LiFeAs as a function of temperature using angle-resolved photoemission spectroscopy. Below approximately 50 K, both the d(yz) hole band at the Z point and the d(xz/yz) electron band at the A point shift to a higher binding energy side. However, at the high-symmetry points Z, A, Gamma, and M, the remaining bands are almost independent of temperature. One of the possible scenarios for these observations is that a strong, three-dimensional orbital-dependent correlation exists in the normal state of LiFeAs in relation to short-range spin fluctuations.

DOI： 10.1103/PhysRevB.93.024503

Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN INST METALS  

The dimensionless figure of merit, ZT, of Al-Mn-Si based C54 phase was effectively increased using the precipitation of a small amount of Al-Mn-Si based C40 phase, which possesses slightly smaller magnitude of both electrical resistivity and Seebeck coefficient than those of C54 phase. We prepared a series of samples by simply alloying Al-37(Mn26Ru3Re3Fe1)Si-30) C54 phase and Al-27.5(Mn29W3Fe1)Si-39.5 C40 phase as [Al-37(Mn26Ru3Re3Fe1)Si-30](1-x)[Al-27.5(Mn29W3Fe1)Si-39.5](x) at x = 0, 0.01, 0.02, 0.03, 0.05, and 1, and found that the magnitude of Seebeck coefficient remained unchanged at x < 0.03, while the lattice thermal conductivity and electrical resistivity were moderately decreased with increasing x. The maximum ZT has consequently increased up to ZT = 0.42 at x = 0.03 from ZT = 0.38 of the C54-phase at x = 0.

DOI： 10.2320/matertrans.MF201613

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

We study the electronic structure of bulk single crystals and epitaxial films of Fe_{3}O_{4}. Fe 2p core level spectra show clear differences between hard x-ray (HAX) and soft x-ray photoemission spectroscopy (PES). The bulk-sensitive spectra exhibit temperature (T) dependence across the Verwey transition, which is missing in the surface-sensitive spectra. By using an extended impurity Anderson full-multiplet model-and in contrast to an earlier peak assignment-we show that the two distinct Fe species (A and B site) and the charge modulation at the B site are responsible for the newly found double peaks in the main peak above T_{V} and its T-dependent evolution. The Fe 2p HAXPES spectra show a clear magnetic circular dichroism (MCD) in the metallic phase of magnetized 100-nm-thick films. The model calculations also reproduce the MCD and identify the contributions from magnetically distinct A and B sites. Valence band HAXPES shows a finite density of states at E_{F} for the polaronic half metal with a remnant order above T_{V} and a clear gap formation below T_{V}. The results indicate that the Verwey transition is driven by changes in the strongly correlated and magnetically active B-site electronic states, consistent with resistivity and optical spectra.

DOI： 10.1103/PhysRevLett.115.256405

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

Strong spin-orbit coupling fosters exotic electronic states such as topological insulators and superconductors, but the combination of strong spin-orbit and strong electron-electron interactions is just beginning to be understood. Central to this emerging area are the 5d transition metal iridium oxides. Here, in the pyrochlore iridate Pr2Ir2O7, we identify a non-trivial state with a single-point Fermi node protected by cubic and time-reversal symmetries, using a combination of angle-resolved photoemission spectroscopy and first-principles calculations. Owing to its quadratic dispersion, the unique coincidence of four degenerate states at the Fermi energy, and strong Coulomb interactions, non-Fermi liquid behaviour is predicted, for which we observe some evidence. Our discovery implies that Pr2Ir2O7 is a parent state that can be manipulated to produce other strongly correlated topological phases, such as topological Mott insulator, Weyl semimetal, and quantum spin and anomalous Hall states.

DOI： 10.1038/ncomms10042

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

A 1D metallic surface state was created on an anisotropic InSb(001) surface covered with Bi. Angle-resolved photoelectron spectroscopy (ARPES) showed a 1D Fermi contour with almost no 2D distortion. Close to the Fermi level (E-F), the angle-integrated photoelectron spectra showed power-law scaling with the binding energy and temperature. The ARPES plot above E-F, obtained thanks to a thermally broadened Fermi edge at room temperature, showed a 1D state with continuous metallic dispersion across E-F and power-law intensity suppression around E-F. These results strongly suggest a Tomonaga-Luttinger liquid on the Bi/InSb(001) surface.

DOI： 10.1103/PhysRevLett.115.256404

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

Transition metal compounds often undergo spin-charge-orbital ordering due to strong electron-electron correlations. In contrast, low-dimensional materials can exhibit a Peierls transition arising from low-energy electron-phonon-coupling-induced structural instabilities. We study the electronic structure of the tunnel framework compound K2Cr8O16, which exhibits a temperature-dependent (T-dependent) paramagnetic-to-ferromagnetic- metal transition at T-C = 180 K and transforms into a ferromagnetic insulator below T-MI = 95 K. We observe clear T-dependent dynamic valence (charge) fluctuations from above T-C to T-MI, which effectively get pinned to an average nominal valence of Cr+3.75 (Cr4+:Cr3+ states in a 3:1 ratio) in the ferromagnetic-insulating phase. High-resolution laser photoemission shows a T-dependent BCS-type energy gap, with 2G(0) similar to 3.5(k(B)T(MI)) similar to 35 meV. First-principles band-structure calculations, using the experimentally estimated on-site Coulomb energy of U similar to 4 eV, establish the necessity of strong correlations and finite structural distortions for driving the metal-insulator transition. In spite of the strong correlations, the nonintegral occupancy (2.25 d-electrons/Cr) and the half-metallic ferromagnetism in the t(2g) up-spin band favor a low-energy Peierls metal-insulator transition.

DOI： 10.1103/PhysRevX.5.041004

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

We have performed the photoemission and inverse photoemission experiments to elucidate the origin of Mott insulating states in A-site ordered perovskite CaCu3Ti4O12 (CCTO). Experimental results have revealed that Cu 3d-O 2p hybridized bands, which are located around the Fermi level in the prediction of the local-density approximation (LDA) band calculations, are actually separated into the upper Hubbard band at similar to 1.5 eV and the lower Hubbard band at similar to-1.7 eV with a band gap of similar to 1.5-1.8 eV. We also observed that Cu 3d peak at similar to-3.8 eV and Ti 3d peak at similar to 3.8 eV are further away from each other than as indicated in the LDA calculations. In addition, it is found that the multiplet structure around similar to 9 eV includes a considerable number of O 2p states. These observations indicate that the Cu 3d and Ti 3d electrons hybridized with the O 2p states are strongly correlated, which originates in the Mott-insulating states of CCTO. (C) 2015 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ssc.2015.05.007

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

We performed high-resolution photon-energy and polarization-dependent ARPES measurements on ultrathin Bi(111) films [6-180 bilayers (BL), 2.5-70 nm thick] formed on Si(111). In addition to the extensively studied surface states (SSs), the edge of the bulk valence band was clearly measured by using S-polarized light. We found direct evidence that this valence band edge, which forms a hole pocket in the bulk Bi crystal, does not cross the Fermi level for the 180 BL thick film. This is consistent with the predicted semimetal-to-semiconductor transition due to the quantum-size effect [V.B. Sandomirskii, Sov. Phys. JETP 25, 101 (1967)]. However, it became metallic again when the film thickness was decreased (below 30 BL). A plausible explanation for this phenomenon is the modification of the charge neutrality condition due to the size effect of the SSs.

DOI： 10.1103/PhysRevLett.115.106803

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

Employing the angle-resolved photoemission spectroscopy, we study the electronic structure of TaFe1.23Te3, a two-leg spin ladder compound with a novel antiferromagnetic ground state. Quasi-two-dimensional (2D) Fermi surface is observed, with sizable inter-ladder hopping. Moreover, instead of observing an energy gap at the Fermi surface in the antiferromagnetic state, we observe the shifts of various bands. Combining these observations with density-functional-theory calculations, we propose that the large scale reconstruction of the electronic structure, caused by the interactions between the coexisting itinerant electrons and local moments, is most likely the driving force of the magnetic transition. Thus TaFe1.23Te3 serves as a simpler platform that contains similar ingredients to the parent compounds of iron-based superconductors.

DOI： 10.1088/0256-307X/32/2/027401

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

Publishing type：Research paper (scientific journal)  

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

The electronic structure of Ni2P(0001) has been investigated by photoelectron spectroscopy (PES) utilizing synchrotron radiation. A Ni 3d-P 3p hybrid band was observed at 1-5 eV, and the band had peaks at 1.5 eV and 3.4 eV together with a shoulder around 2.4 eV. The resonant PES study showed that the Ni 3d component was included in the whole binding energy region of the band. The change in the surface electronic structure of Ni2P(0001) induced by the segregation of P atoms was investigated by annealing temperature-dependent measurements of PES, and it was found that the change proceeded via following two steps: the segregation of P atoms proceeded at about 200 degrees C, and the bonding between surface Ni atoms and segregated P atoms was facilitated at > 300 degrees C. The bonding between Ni and P atoms induced substantial stabilization of Ni 3d levels, which is contrary to the case of Fe2P(0001) where the segregation of P atoms induced little change in DOS of Fe 3d levels.

DOI： 10.1380/ejssnt.2015.93

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

We report angle-resolved photoemission spectroscopy, electrical resistivity and Hall effect measurements on Sm1-xYxS. At the smallest doping concentration x = 0.03, the system changes from a semiconducting to metallic conductivity, whereas it is at a critical concentration x(c) similar or equal to 0.19 that there occurs the volume collapse accompanied by a color change from black to golden. It appears that the band gap between 4f and 5d bands vanishes at x(c). From these results, we suggest that the black-to-golden transition of Sm1-xYxS has the same mechanism as the pressure-induced valence transition of SmS, but the semiconductor-to-metal transition has a different origin for Sm1-xYxS and SmS.

DOI： 10.1088/1742-6596/592/1/012028

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

Time-resolved hard x-ray photoelectron spectroscopy (trHAXPES) is established using the x-ray free-electron laser SACLA. The technique extends time-resolved photoemission into the hard x-ray regime and, as a core-level spectroscopy, combines element and atomic-site specificity and sensitivity to the chemical environment with femtosecond time resolution and bulk (sub-surface) sensitivity. The viability of trHAXPES using 8 keV x-ray free-electron-laser radiation is demonstrated by a systematic investigation of probe and pump pulse-induced vacuum space-charge effects on the V 1s emission of VO2 and the Ti 1s emission of SrTiO3. The time and excitation energy dependencies of the measured spectral shifts and broadenings are compared to the results of N-body numerical simulations and simple analytic (mean-field) models. Good agreement between the experimental and calculated results is obtained. In particular, the characteristic temporal evolution of the pump pulse-induced spectral shift is shown to provide an effective means to determine the temporal overlap of pump and probe pulses. trHAXPES opens a new avenue in the study of ultrafast atomic-site specific electron and chemical dynamics in materials and at buried interfaces.

DOI： 10.1088/1367-2630/16/12/123045

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

We observed the anisotropic superconducting-gap (SC-gap) structure of a slightly overdoped superconductor, Ba(Fe1-xCox)(2)As-2 (x = 0.1), using three-dimensional (3D) angle-resolved photoemission spectroscopy. Two hole Fermi surfaces (FSs) observed at the Brillouin zone center and an inner electron FS at the zone corner showed a nearly isotropic SC gap in 3D momentum space. However, the outer electron FS showed an anisotropic SC gap with nodes or gap minima around the M and A points. The different anisotropies obtained the SC gap between the outer and inner electron FSs cannot be expected from all theoretical predictions with spin fluctuation, orbital fluctuation, and both competition. Our results provide a new insight into the SC mechanisms of iron pnictide superconductors.

DOI： 10.7566/JPSJ.83.093703

Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN INST METALS & MATERIALS  

We investigated the electronic and local crystal structures of the sintered half-Heusler ZrNiSn alloy by synchrotron radiation photoemission spectroscopy (SR-PBS), synchrotron radiation X-ray powder diffraction (SR-XRD) measurements, and electronic band structure calculations to clarify mechanisms leading to improvements in the thermoelectric properties of materials. In contrast to the predicted semiconductor-like electronic structure, the SR-PES results show a pseudo-gap at the Fermi level, and the SR-XRD analysis reveals an interstitial Ni disorder in the half-Heusler structure. An improvement in the thermoelectric properties can be achieved by material design based on the pseudo-gap electronic structure of half-Heusler ZrNiSn-based alloys.

DOI： 10.2320/matertrans.E-M2014803

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

We study the electronic structure of CeCuAs2, which is known to show an anomalous negative temperature coefficient of resistivity below room temperature, while the thermopower shows metallic behavior. We carry out hard x-ray photoemission spectroscopy (PES), x-ray absorption spectroscopy (XAS), and resonant PES across the Ce 3d-4f and Cu 2p-3d thresholds to investigate the role of Kondo screening in CeCuAs2. The Ce 3d core-level PES shows the f0, f1, and f2 features, and the Ce 3d-4f XAS shows corresponding features due to transitions into the f1 and f2 states, as in Kondo systems. The spectral feature assignments are confirmed by single-impurity Anderson model calculations and indicate a nearly trivalent-Ce configuration with small but finite mixed valency. The resonant PES valence band spectra across the Ce 3d-4f threshold show an intense Ce 4f1 resonance just below the Fermi level, while the Ce 4f0 feature is observed at a binding energy of 2.5 eV. The obtained f partial density of states shows an unusual pseudogaplike dip at the Fermi level in CeCuAs2. The results indicate the importance of Kondo screening and a pseudogap in the f partial density of states for the anomalous transport and magnetic properties of CeCuAs2. © 2014 American Physical Society.

DOI： 10.1103/PhysRevB.89.235117

Language：English   Publishing type：Research paper (other academic)   Publisher：JPS  

DOI： 10.7566/JPSCP.3.011080

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

We study the electronic structure of CeCuAs2, which is known to show an anomalous negative temperature coefficient of resistivity below room temperature, while the thermopower shows metallic behavior. We carry out hard x-ray photoemission spectroscopy (PES), x-ray absorption spectroscopy (XAS), and resonant PES across the Ce 3d-4f and Cu 2p-3d thresholds to investigate the role of Kondo screening in CeCuAs2. The Ce 3d core-level PES shows the f(0), f(1), and f(2) features, and the Ce 3d-4f XAS shows corresponding features due to transitions into the f(1) and f(2) states, as in Kondo systems. The spectral feature assignments are confirmed by single-impurity Anderson model calculations and indicate a nearly trivalent-Ce configuration with small but finite mixed valency. The resonant PES valence band spectra across the Ce 3d-4f threshold show an intense Ce 4f(1) resonance just below the Fermi level, while the Ce 4f(0) feature is observed at a binding energy of 2.5 eV. The obtained f partial density of states shows an unusual pseudogaplike dip at the Fermi level in CeCuAs2. The results indicate the importance of Kondo screening and a pseudogap in the f partial density of states for the anomalous transport and magnetic properties of CeCuAs2.

DOI： 10.1103/PhysRevB.89.235117

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

The electronic structure of Fe2P(0001) has been investigated by photoelectron spectroscopy (PES) utilizing synchrotron radiation. In the valence band (VB) spectra, an Fe 3d-P 3p hybrid band (main band) was observed at 0-3 eV independent of photon energy, and satellites were observed at 4.5 and 7 eV at hv &lt; 55 eV. The satellites showed a resonant behavior around the Fe 3p threshold, and are associated with the photoemission process leading to two-hole bound final states. The slight Ar+ ion sputtering (1 kV, 5 min) induced little change in VB spectra, while core-level PES study showed that the P atoms in the surface region were selectively removed. This suggests that the stabilization of the metal sites due to the bonding with P atoms is ineffective on Fe2P(0001), unlike the situation reported earlier for Ni2P. (C) 2014 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.susc.2014.01.012

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

DOI： 10.7566/JPSCP.3.015028

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

The electronic structure of a quantum spin liquid compound, EtMe3Sb[Pd(dmit)2]2, has been studied with angle-resolved photoemission spectroscopy, together with two other Pd(dmit)2 salts in the valence bond solid or antiferromagnetic state. EtMe3Sb[Pd(dmit)2]2, being a Mott insulator, has its lower Hubbard band identified, with a soft energy gap at the chemical potential. In addition, the spectral features exhibit anomalously broad linewidth and negligible dispersion, which fit well to the calculated energy levels of an isolated [Pd(dmit)2]2 dimer. We suggest that these electronic characteristics might be the manifestation of the spin-charge separation in a two-dimensional quantum spin liquid. © 2014 American Physical Society.

DOI： 10.1103/PhysRevB.89.075105

Language：English   Publishing type：Research paper (scientific journal)   Publisher：INT UNION CRYSTALLOGRAPHY  

In order to utilize high-brilliance photon sources, such as X-ray free-electron lasers (XFELs), for advanced time-resolved photoelectron spectroscopy (TR-PES), a single-shot CCD-based data acquisition system combined with a high-resolution hemispherical electron energy analyzer has been developed. The system's design enables it to be controlled by an external trigger signal for single-shot pump-probe-type TR-PES. The basic performance of the system is demonstrated with an offline test, followed by online core-level photoelectron and Auger electron spectroscopy in 'single-shot image', 'shot-to-shot image (image-to-image storage or block storage)' and 'shot-to-shot sweep' modes at soft X-ray undulator beamline BL17SU of SPring-8. In the offline test the typical repetition rate for image-to-image storage mode has been confirmed to be about 15 Hz using a conventional pulse-generator. The function for correcting the shot-to-shot intensity fluctuations of the exciting photon beam, an important requirement for the TR-PES experiments at FEL sources, has been successfully tested at BL17SU by measuring Au 4f photoelectrons with intentionally controlled photon flux. The system has also been applied to hard X-ray PES (HAXPES) in 'ordinary sweep' mode as well as shot-to-shot image mode at the 27 m-long undulator beamline BL19LXU of SPring-8 and also at the SACLA XFEL facility. The XFEL-induced Ti 1s core-level spectrum of La-doped SrTiO3 is reported as a function of incident power density. The Ti 1s core-level spectrum obtained at low power density is consistent with the spectrum obtained using the synchrotron source. At high power densities the Ti 1s core-level spectra show space-charge effects which are analysed using a known mean-field model for ultrafast electron packet propagation. The results successfully confirm the capability of the present data acquisition system for carrying out the core-level HAXPES studies of condensed matter induced by the XFEL.

DOI： 10.1107/S1600577513028233

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

We have realized element-specific pump-probe time-resolved hard X-ray photoelectron spectroscopy (TR-HAXPES) for the study of ultrafast electron dynamics in condensed matter. TR-HAXPES has been applied for the first time to determine the temporal evolution of space-charge effects on Ti 1s core-level photoelectron spectra of SrTiO₃ at the SACLA X-ray free electron laser facility. We found that the temporal evolution of Ti 1s kinetic energy shifts can be well explained by a mean-field model for the electron propagation in the vacuum. We also report preliminary results of a real-time laser-assisted valence-transition in YbInCu₄, observed by measuring Yb 3d_5/2 core-level HAXPES spectra at BL19LXU of SPring-8, under ON/OFF conditions of an optical pump-laser. We observed drastic changes in intensities of the Yb²⁺ and Yb³⁺ spectral components induced by pump-laser irradiation.

DOI： 10.14723/tmrsj.39.469

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

We report the electronic structure of EuO thin films not only parallel to but also perpendicular to the (0 0 1) plane using synchrotron radiation angle-resolved photoemission spectroscopy. As a result, the electronic structure perpendicular to the (0 0 1) plane is consistent with in-plane electronic structure in spite of the thin films. This result implies that the electronic structure of EuO thin films is consistent with that of the bulk materials. (C) 2013 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.elspec.2013.10.008

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

Topological insulators are insulating materials but have metallic edge states with peculiar properties. They are considered to be promising for the development of future low energy consumption nano-electronic devices. However, there is a major problem: Naturally grown materials are not truly insulating owing to defects in their crystal structure. In the present study, we have examined the electronic structure and transport properties of topological insulator ultrathin Bi2Te3 films by angle-resolved photoemission spectroscopy and in situ transport measurements. To realize a truly bulk insulating film, we tried to tune the Fermi-level position using two methods. The first of these, i.e., changing the Si substrate temperature during film growth (350-450 K) to reduce the defects in the grown films, had some effect in reducing the bulk residual carriers, but we could not fabricate a film that showed only the surface states crossing the Fermi level. The second method we employed was to incorporate Pb atoms during film growth since Pb has one less electron than Bi. When the films were grown at around 350 K, we observed a systematic shift in the Fermi level and obtained a bulk insulating film, although it was not possible to move the Dirac point just at the Fermi level. The change in the measured film conductivity was consistent with the shift in the Fermi level and suggested the detection of the surface-state conductivity. For films grown at a higher substrate temperature (450 K), the Fermi level could be tuned only slightly and a bulk n-type film was obtained. Pb incorporation changes the shape of the Dirac cone, suggesting the formation of a stoichiometric ternary alloy of Bi, Pb, and Te, which is another topological insulator. (C) 2013 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.52.110112

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

We report angle-resolved photoemission spectroscopy results of A-site ordered perovskite CaCu3Ti4O12. We have observed the clear band dispersions, which are shifted to the higher energy by 1.7 eV and show the band narrowing around 2 eV in comparison with the local density approximation calculations. In addition, the high-energy multiplet structures of Cu 3d(8) final states have been found around 8-13 eV. These results reveal that CaCu3Ti4O12 is a Mott-type insulator caused by the strong correlation effects of the Cu 3d electrons well hybridized with O 2p states. Unexpectedly, there exists a very small spectral weight at the Fermi level in the insulator phase, indicating the existence of isolated metallic states.

DOI： 10.1103/PhysRevB.88.205133

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

The microscopic mechanism for Rashba-type band splitting is examined in detail. We show how an asymmetric charge distribution is formed when the local orbital angular momentum (OAM) and crystal momentum get interlocked due to surface effects. An electrostatic energy term in the Hamiltonian appears when such an OAM-and crystal-momentum-dependent asymmetric charge distribution is placed in an electric field produced by inversion-symmetry breaking. Analysis by using an effective Hamiltonian shows that, as the atomic spin-orbit coupling (SOC) strength increases from weak to strong, the originally OAM-quenched states evolve into well-defined chiral OAM states and then to states of total angular momentum J. In addition, the energy scale of the band splitting changes from the atomic SOC energy to electrostatic energy. To confirm the validity of the model, we study OAM and spin structures of the Au(111) system by using an effective Hamiltonian for the d-orbital case. As for the strong-SOC regime, we choose Bi2Te2Se as a prototype system. We performed circular dichroism angle-resolved photoemission spectroscopy experiments as well as first-principles calculations. We find that the effective model can explain various aspects of the spin and OAM structures of the system.

DOI： 10.1103/PhysRevB.88.205408

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

Electron-phonon coupling (EPC) plays an important role in solid state physics. Here, we demonstrate an experimental method that enables investigation of the elemental processes of the indirect transition, in which EPC participates in photoexcitation in solids, by resolving the energy and momentum of phonons and electrons simultaneously. For graphite, we used angle-resolved photoelectron spectroscopy to observe electron emission at the Gamma-point being scattered from the K-point by a phonon. Energy conservation during phonon emission implies that the step-like structure in the spectrum is near the Fermi level, and angle-resolved measurements revealed phonon dispersions that contribute to EPC because of parallel momentum conservation. The observed phonon branch depends on the photon energy, i.e., the final photoexcitation state; this dependency is partly explained by the selection rule, which is determined by the electron state symmetry for the initial, intermediate, and final states and the phonon.

DOI： 10.1038/srep03031

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

We report the results of angle-resolved photoemission spectroscopy (ARPES) on the Sm1-x Y (x) S system, which exhibits a black-to-golden phase transition induced by yttrium substitution and by an applied external pressure, in order to investigate the mechanism of this transition and the electronic structure of both phases. The obtained ARPES spectrum of pure SmS (x = 0) indicates a band insulator, in which the shallowest electronic state is the Sm2+ 4f state at a binding energy of about 0.4 eV. With increasing x, the top of the Sm2+ 4f band shifts to the lower binding energy side and touches the Fermi level for x = 0.33. Because of the discontinuous reduction of the lattice constant in the vicinity of the black-to-golden phase transition (x similar to 0.17), the golden phase induced by yttrium substitution is concluded to be an intermediate valence state.

DOI： 10.3938/jkps.62.2028

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

DOI： 10.1103/PhysRevLett.110.149901

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

We report the electronic structure of a prototypical valence fluctuation system, YbAl2, using angle-resolved photoemission spectroscopy. The observed band dispersions and Fermi surfaces are well described in terms of band structure calculations based on local density approximation. Strong hybridization between the conduction and 4f bands is identified on the basis of the periodic Anderson model. The evaluated small mass enhancement factor and the high Kondo temperature qualitatively agree with those obtained from thermodynamic measurements. Such findings suggest that the strong hybridization suppresses band renormalization and is responsible for the valence fluctuations in YbAl2.

DOI： 10.1103/PhysRevB.87.165141

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

The tunability of bonding character in transition-metal compounds controls phase transitions and their fascinating properties such as high-temperature superconductivity, colossal magnetoresistance, spin-charge ordering, etc. However, separating out and quantifying the roles of covalency and metallicity derived from the same set of transition-metal d and ligand p electrons remains a fundamental challenge. In this study, we use bulk-sensitive photoelectron spectroscopy and configuration-interaction calculations for quantifying the covalency and metallicity in correlated compounds. The method is applied to study the first-order temperature- (T-) dependent metal-insulator transitions (MITs) in the cubic pyrochlore ruthenates Tl2Ru2O 7 and Hg2Ru2O7. Core-level spectroscopy shows drastic T-dependent modifications which are well explained by including ligand-screening and metallic-screening channels. The core-level metallic-origin features get quenched upon gap formation in valence band spectra, while ionic and covalent components remain intact across the MIT. The results establish temperature-driven Mott-Hubbard MITs in three-dimensional ruthenates and reveal three energy scales: (a) 4d electronic changes occur on the largest (∼eV) energy scale, (b) the band-gap energies/charge gaps (Eg∼160-200 meV) are intermediate, and (c) the lowest-energy scale corresponds to the transition temperature TMIT (∼10 meV), which is also the spin gap energy of Tl2Ru2O7 and the magnetic-ordering temperature of Hg2Ru2O 7. The method is general for doping- and T-induced transitions and is valid for V2O3, CrN, La1-xSr xMnO3, La2-xSrxCuO4, etc. The obtained transition-metal-ligand (d-p) bonding energies (V∼45-90 kcal/mol) are consistent with thermochemical data, and with energies of typical heteronuclear covalent bonds such as C-H, C-O, C-N, etc. In contrast, the metallic-screening energies of correlated compounds form a weaker class (V *∼10-40 kcal/mol) but are still stronger than van der Waals and hydrogen bonding. The results identify and quantify the roles of covalency and metallicity in 3d and 4d correlated compounds undergoing metal-insulator transitions. © 2013 American Physical Society.

DOI： 10.1103/PhysRevB.87.045108

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

The tunability of bonding character in transition-metal compounds controls phase transitions and their fascinating properties such as high-temperature superconductivity, colossal magnetoresistance, spin-charge ordering, etc. However, separating out and quantifying the roles of covalency and metallicity derived from the same set of transition-metal d and ligand p electrons remains a fundamental challenge. In this study, we use bulk-sensitive photoelectron spectroscopy and configuration-interaction calculations for quantifying the covalency and metallicity in correlated compounds. The method is applied to study the first-order temperature(T-) dependent metal-insulator transitions (MITs) in the cubic pyrochlore ruthenates Tl2Ru2O7 and Hg2Ru2O7. Core-level spectroscopy shows drastic T-dependent modifications which are well explained by including ligand-screening and metallic-screening channels. The core-level metallic-origin features get quenched upon gap formation in valence band spectra, while ionic and covalent components remain intact across the MIT. The results establish temperature-driven Mott-Hubbard MITs in three-dimensional ruthenates and reveal three energy scales: (a) 4d electronic changes occur on the largest (similar to eV) energy scale, (b) the band-gap energies/charge gaps (E-g similar to 160-200 meV) are intermediate, and (c) the lowest-energy scale corresponds to the transition temperature TMIT (similar to 10 meV), which is also the spin gap energy of Tl2Ru2O7 and the magnetic-ordering temperature of Hg2Ru2O7. The method is general for doping-and T-induced transitions and is valid for V2O3, CrN, La1-xSrxMnO3, La2-xSrxCuO4, etc. The obtained transition-metal-ligand (d-p) bonding energies (V similar to 45-90 kcal/mol) are consistent with thermochemical data, and with energies of typical heteronuclear covalent bonds such as C-H, C-O, C-N, etc. In contrast, the metallic-screening energies of correlated compounds form a weaker class (V* similar to 10-40 kcal/mol) but are still stronger than van der Waals and hydrogen bonding. The results identify and quantify the roles of covalency and metallicity in 3d and 4d correlated compounds undergoing metal-insulator transitions. DOI: 10.1103/PhysRevB.87.045108

DOI： 10.1103/PhysRevB.87.045108

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

Mini-bands formed in superlattice structures are often used for an intermediate-band solar cell. In order to design an optimized structure, it is important to measure the mini-band dispersions which influence transport properties and solar-energy conversion efficiency. In this study, we directly observed the dispersions of mini-bands formed in InGaAs/AlGaAs quantum-well superlattices by angle-resolved photoemission spectroscopy with synchrotron radiation light. The short-period energy dispersions due to the mini-bands around the X-valley were clearly obtained. In addition, the effective mass coefficient of each mini-band could be individually evaluated.

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

We studied the atomic and electronic structures of ultrathin Bi(111) films grown on Bi2Te3 by means of angle-resolved photoemission, first-principles calculations, and low-energy electron diffraction. These Bi films were found to be strained due to the influence of the substrate. Accordingly, the band structure is affected and Bi undergoes a topological phase transition; it is shown that the Z(2) topological invariant in three dimensions switches from +1 (trivial) to -1 (nontrivial or topological). This was clearly confirmed from the change in the surface-state dispersion near the Fermi level. Our discovery offers a method to produce novel topological systems from simple materials.

DOI： 10.1103/PhysRevLett.109.227401

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

The newly discovered iron-chalcogenide superconductor K (x) Fe2-y Se-2 exhibits a distinct electronic structure from other iron-based superconductors. Exploiting polarization-dependent angle-resolved photoemission spectroscopy, we have determined the orbital characters of band structure in a K (x) Fe2-y Se-2 superconductor. To a large extent, we find that K (x) Fe2-y Se-2 superconductor shares similar orbital characters with other iron-based superconductors, but with its own characteristics. For example, we have resolved two highly degenerate electron cylinders around the zone corner in the s and p geometries, respectively, indicating negligible interactions between them. Moreover, in contrast to the band calculation results, the small electron pocket around Z is found to be mainly consisted of the d (Z) orbital. The determined orbital characters would help to construct a realistic model for K (x) Fe2-y Se-2.

DOI： 10.1007/s11434-012-5405-7

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

We use hard x-ray photoemission spectroscopy (HAXPES) to investigate the electronic structure of YbAl2, for which the Yb valence has not been consistently reported to date. The bulk sensitivity and the analytical simplicity provided by the Yb 3d core-level HAXPES allow a reliable determination of the mean valence of Yb ions. For YbAl2, it is evaluated to be +2.20, which remains nearly unchanged below 300 K. The Kondo resonance peak with an extremely high Kondo temperature (above 2000 K) is clearly identified in the valence-band spectra. The results indicate that a coherent Kondo state can be robust even in a nearly divalent system.

DOI： 10.1143/JPSJ.81.073702

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

Optical conductivity [σ(ω)] of PrRu 4P 12 has been studied under high pressure to 14 GPa, at low temperatures to 8 K, and at photon energies 12 meV-1.1 eV. The energy gap in σ(ω) at ambient pressure, caused by a metal-insulator transition due to an unconventional charge-density-wave formation at 63 K, is gradually filled in with increasing pressure to 10 GPa. At 14 GPa and below 30 K, σ(ω) exhibits a pronounced Drude-type component due to free carriers. This indicates that the initial insulating ground state at zero pressure has been turned into a metallic one at 14 GPa. This is consistent with a previous resistivity study under pressure, where the resistivity rapidly decreased with cooling below 30 K at 14 GPa. The evolution of electronic structure with pressure is discussed in terms of the hybridization between the 4f and conduction electrons. © 2012 American Physical Society.

DOI： 10.1103/PhysRevB.85.205116

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

DOI： 10.1088/0953-8984/24/15/159501

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

We report the three-dimensional electronic structure of iron pnictide superconductor LiFeAs obtained by polarization-dependent angle-resolved photoemission spectroscopy. The obtained orbital characters of each Fermi surface (FS) as well as the band dispersions were qualitatively consistent with those derived from local density approximation band calculations. It was found that FS nesting appears between a two-dimensional hole band at the zone center and an electron band at the zone corner with the same d xy orbital character. A shadow band attributed to (π,π,π) band folding was also observed without the spin-density-wave transition. This result suggests that FS nesting between bands with the same orbital d xy character due to spin fluctuation plays an important role in LiFeAs. © 2012 American Physical Society.

DOI： 10.1103/PhysRevB.85.094509

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

We report the three-dimensional electronic structure of iron pnictide superconductor LiFeAs obtained by polarization-dependent angle-resolved photoemission spectroscopy. The obtained orbital characters of each Fermi surface (FS) as well as the band dispersions were qualitatively consistent with those derived from local density approximation band calculations. It was found that FS nesting appears between a two-dimensional hole band at the zone center and an electron band at the zone corner with the same d(xy) orbital character. A shadow band attributed to (pi,pi,pi) band folding was also observed without the spin-density-wave transition. This result suggests that FS nesting between bands with the same orbital d(xy) character due to spin fluctuation plays an important role in LiFeAs.

DOI： 10.1103/PhysRevB.85.094509

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

We report the new fabrication method of single crystalline EuO thin film along the (100) plane and its crystal structure and magnetic property. EuO thin film was fabricated on SrO buffer layer created on SrTiO3 substrate. The obtained EuO thin film was formed a single phase of the rock-salt structure with the lattice constant of 0.516 nm and showed ferromagnetism below the Curie temperature of 71 K. The examined physical properties are consistent with the bulk materials.

DOI： 10.1088/1742-6596/391/1/012047

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

We report the orbital character of the Fermi surfaces for the Fe-based superconductor LiFeAs using the polarization-dependent three-dimensional angle-resolved photoemission spectroscopy and the angular-dependent dipole selection rule analysis. The evaluated orbital character is well reproduced by the band calculation. The results demonstrate that the polarization-dependent ARPES combined with the selection rule analysis is useful to identify the orbital character of the Fe-based unconventional superconductivity.

DOI： 10.1088/1742-6596/391/1/012125

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

We performed angle-resolved photoemission (ARPES) experiments on Bi2Te3 with circularly polarized light. ARPES data show very strong circular dichroism, indicating the existence of orbital angular momentum (OAM). Moreover, the alignment of OAMis found to have a strong binding energy dependence. Such energy dependence comes from a relatively strong band warping effect in Bi2Te3 compared to Bi2Se3. OAM close to the Dirac point has an ideal chiral structure (sin.) without an out-of-plane component. The warping effect comes in as the binding energy decreases, and circular dichroism along a constant energy contour can no longer be explained by a simple sin theta function but requires a sin 3 theta term. When the warping effect becomes even stronger near the Fermi energy, circular dichroism gains an additional sin 6 theta term. Such behavior is found to be compatible with the theoretically predicted OAM structure.

DOI： 10.1103/PhysRevB.84.245435

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

The Kondo resonance at the Fermi level is well established for the electronic structure of Ce (4f(1) electron) and Yb (4f(1) hole)-based systems. In this work, we report complementary experimental and theoretical studies on the Kondo resonance in the Pr-based 4f(2) system, PrTi2Al20. Using Pr 3d-4f resonant photoemission spectroscopy and single-impurity Anderson model (SIAM) calculations including the full multiplets of Pr ions, we show that a 4f(2) system can also give rise to a Kondo resonance at the Fermi level. The Kondo resonance peak is experimentally observed through a final-state multiplet-dependent resonance and is reproduced with properly tuned hybridization strength in SIAM calculations.

DOI： 10.1103/PhysRevB.84.193101

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

Soft x-ray photoelectron spectroscopy was performed for alpha-TDAE-C-60 single crystal. A C 1s spectrum with a shakeup satellite of TDAE-C-60 is very similar to that of C-60 when we shift the energy, indicating a charge transfer to the C-60 site. A comparison of a N 1s spectrum with theoretical calculations indicates that TDAE mainly consists of the TDAE(+) state. The valence-band spectrum near the Fermi edge also suggests a charge transfer from the TDAE to the C-60 cage. Based on these results we propose a modified model for the charge transfer: the charge states of C-60(-) and TDAE(+), and the dimer formation along the c axis between the TDAE(+) states.

DOI： 10.1103/PhysRevB.84.161404

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

The anisotropic electronic structure responsible for the antiferromagnetic transition in CeRu2Al10 at the unusually high temperature of T-0 = 28 K was studied using optical conductivity spectra, Ce 3d x-ray photoemission spectra, and band calculation. It was found that the electronic structure in the ac plane is that of a Kondo semiconductor, whereas that along the b axis has a nesting below 32 K (slightly higher than T-0). These characteristics are the same as those of CeOs2Al10 [S. Kimura et al., Phys. Rev. Lett. 106, 056404 (2011)]. The c-f hybridization intensities between the conduction and 4f electrons of CeRu2Al10 and CeOs2Al10 are weaker than that of CeFe2Al10, showing no magnetic ordering. These results suggest that the electronic structure with one-dimensional weak c-f hybridization along the b axis combined with two-dimensional strong hybridization in the ac plane causes charge-density wave (CDW) instability, and the CDW state then induces magnetic ordering.

DOI： 10.1103/PhysRevB.84.165125

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

The angle-resolved photoelectron spectroscopy with low-photon energy (7-16 eV) is used for the investigation of highly oriented pyrolytic graphite at a low temperature. We observed for the first time peaks in the low binding energy region (from the Fermi level to 0.7 eV) of the surface normal photoelectron spectra at 11K, which disappear above similar to 30K. Based on the dispersion both along the parallel and normal to the surface, the peaks are ascribed to the emission from the K(H)-point that is backfolded into the Gamma(A)-point as a result of the two-dimensional superperiodicity. The surface charge density wave transition is proposed for the driving mechanism of the superperiodicity based on the temperature dependence of the photoelectron intensity.

DOI： 10.1103/PhysRevB.84.121411

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

Optical conductivity [sigma(omega)] of Ce-filled skutterudite CeRu4Sb12 has been measured at high pressure to 8 GPa and at low temperature, to probe the pressure evolution of its electronic structures. At ambient pressure, a mid-infrared peak at 0.1 eV was formed in sigma(omega) at low temperature, and the spectral weight below 0.1 eV was strongly suppressed, due to a hybridization of the f electron and conduction electron states. With increasing external pressure, the mid-infrared peak shifts to higher energy, and the spectral weight below the peak was further depleted. The obtained spectral data are analyzed in comparison with band calculation result and other reported physical properties. It is shown that the electronic structure of CeRu4Sb12 becomes similar to that of a narrow-gap semiconductor under external pressure.

DOI： 10.1143/JPSJ.80.084718

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

Infrared reflectance spectroscopy has been used to probe the pressure evolution of metal-insulator (MI) transition in PrRu4P12 up to 14 GPa. The decrease of reflectance at low temperatures, which is due to an energy gap associated with the MI transition, is almost unchanged with pressure up to 10 GPa. This decrease of reflectance, however, is strongly suppressed at 14 GPa, and an increase of reflectance is observed below 30 K. This result indicates that the MI transition of PrRu4P12 has been suppressed at 14 GPa.

DOI： 10.1143/JPSJS.80SA.SA092

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

We report on the electronic structure of the perovskite oxide CaCrO3 using valence-band, core-level, and Cr 2p-3d resonant photoemission spectroscopy (PES). Despite its antiferromagnetic order, a clear Fermi edge characteristic of a metal with dominant Cr 3d character is observed in the valence-band spectrum. The Cr 3d single-particle density of states are spread over 2 eV, with the photoemission spectral weight distributed in two peaks centered at similar to 1.2 and 0.2 eV below E-F, suggestive of the coherent and incoherent states resulting from strong electron-electron correlations. Resonant PES across the Cr 2p-3d threshold identifies a "two-hole" correlation satellite and yields an on-site Coulomb energy U similar to 4.8 eV. The metallic DOS at E-F is also reflected through the presence of a well-screened feature at the low binding energy side of the Cr 2p core-level spectrum. X-ray-absorption spectroscopy at Cr L-3,L-2 and O K edges exhibit small temperature-dependent changes that point toward a small change in Cr-O hybridization. The Cr 2p core-level spectrum can be reproduced using cluster model calculations that include a charge transfer from the metallic screening channel at E-F. The overall results indicate that CaCrO3 is a strongly hybridized antiferromagnetic metal, lying in the regime intermediate to Mott-Hubbard and charge-transfer systems.

DOI： 10.1103/PhysRevB.83.165132

Publishing type：Research paper (scientific journal)  

We performed an x-ray spectroscopic study combining resonant x-ray emission spectroscopy (RXES) and photoelectron spectroscopy on the superconducting ternary silicide YbGa1.15Si0.85 and nonsuperconducting ternary germanide YbGaxGe2-x (x=1.0 and 1.1). The Yb valence for all three compounds is found to be about 2.3. In YbGa 1.15Si0.85 no temperature dependence of the Yb valence is observed in the RXES spectra in the temperature range of 7-300 K, while the valence shows a drastic increase under pressure from the Yb2+ state partially including itinerant electrons to the localized Yb3+ state. Differences are observed in the valence-band spectra of the photoelectron spectroscopy between YbGa1.15Si0.85 and YbGa xGe2-x, which may be attributed to the difference of crystal structure. We conclude that both the crystal structure of the planar GaSi layer in YbGa1.15Si0.85 and the resultant electronic structure may have a crucial role in the occurrence of superconductivity. © 2011 American Physical Society.

DOI： 10.1103/PhysRevB.83.104525

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

We performed an x-ray spectroscopic study combining resonant x-ray emission spectroscopy (RXES) and photoelectron spectroscopy on the superconducting ternary silicide YbGa1.15Si0.85 and nonsuperconducting ternary germanide YbGaxGe2-x (x = 1.0 and 1.1). The Yb valence for all three compounds is found to be about 2.3. In YbGa1.15Si0.85 no temperature dependence of the Yb valence is observed in the RXES spectra in the temperature range of 7-300 K, while the valence shows a drastic increase under pressure from the Yb2+ state partially including itinerant electrons to the localized Yb3+ state. Differences are observed in the valence-band spectra of the photoelectron spectroscopy between YbGa1.15Si0.85 and YbGaxGe2-x, which may be attributed to the difference of crystal structure. We conclude that both the crystal structure of the planar GaSi layer in YbGa1.15Si0.85 and the resultant electronic structure may have a crucial role in the occurrence of superconductivity.

DOI： 10.1103/PhysRevB.83.104525

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

Time-resolved photoelectron spectroscopy (trPES) can directly detect transient electronic structure, thus bringing out its promising potential to clarify nonequilibrium processes arising in condensed matters. Here we report the result of core-level trPES on 1T-TaS2, realized by developing a high-intensity 60-eV laser obtained by high-order harmonic generation. Ta4f core-level trPES offers the transient amplitude of the charge-density wave (CDW), via the site-selective and real-time observation of Ta electrons. The present result indicates an ultrafast photoinduced melting and recovery of CDW amplitude, followed by a peculiar long-life oscillation (i.e., collective amplitudon excitation) accompanying the transfer of electrons among adjacent Ta atoms. core-level trPES offers a broad range of opportunities for investigating the ultrafast atom-specific electron dynamics in photorelated phenomena of interest.

DOI： 10.1103/PhysRevB.83.081104

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

We study the surface and bulk electronic structure of the room-temperature ferromagnet Co∶TiO(2) anatase films using soft- and hard-x-ray photoemission spectroscopy with probe sensitivities of ∼1 and ∼10  nm, respectively. We obtain direct evidence of metallic Ti(3+) states in the bulk, which get suppressed to give a surface semiconductor, thus indicating the difference in electronic structure between surface and bulk. X-ray absorption and resonant photoemission spectroscopy reveal Ti(3+) electrons at the Fermi level (E(F)) and high-spin Co(2+) electrons occurring away from E(F). The results show the importance of the charge neutrality condition: Co(2+)+V(O)(2-)+2Ti(4+)↔Co(2+)+2Ti(3+) (V(O) is oxygen vacancy), which gives rise to the elusive Ti 3d carriers mediating ferromagnetism via the Co 3d-O 2p-Ti 3d exchange interaction pathway of the occupied orbitals.

DOI： 10.1103/PhysRevLett.106.047602

Language：Japanese   Publishing type：Research paper (other academic)  

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

Pairing symmetry is a fundamental property that characterizes a superconductor. For the iron-based high-temperature superconductors, an s ±-wave pairing symmetry has received increasing experimental and theoretical support. More specifically, the superconducting order parameter is an isotropic s-wave type around a particular Fermi surface, but it has opposite signs between the hole Fermi surfaces at the zone centre and the electron Fermi surfaces at the zone corners. Here we report the low-energy electronic structure of the newly discovered superconductors, Ax Fe2 Se 2 (A=K,Cs) with a superconducting transition temperature (Tc) of about 30K. We found Ax Fe2 Se2 (A=K,Cs) is the most heavily electron-doped among all iron-based superconductors. Large electron Fermi surfaces are observed around the zone corners, with an almost isotropic superconducting gap of ∼10.3meV, whereas there is no hole Fermi surface near the zone centre, which demonstrates that interband scattering or Fermi surface nesting is not a necessary ingredient for the unconventional superconductivity in iron-based superconductors. Thus, the sign change in the s ± pairing symmetry driven by the interband scattering as suggested in many weak coupling theories becomes conceptually irrelevant in describing the superconducting state here. A more conventional s-wave pairing is probably a better description. © 2011 Macmillan Publishers Limited. All rights reserved.

DOI： 10.1038/nmat2981

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

We report on the electronic structure and Fermi surfaces of the transition-metal dichalcogenide 1 T-VS2 in the low-temperature charge-density-wave (CDW) ordered phase. Using soft x-ray angle-resolved photoemission spectroscopy (ARPES), we investigate the in-plane and out-of-plane vanadium- and sulfur-derived band dispersions and identify kz dispersions in this layered system. Core-level photoemission and x-ray absorption spectroscopy show that vanadium electrons are in the d1 configuration while 2p-3d resonant ARPES shows only 3d -derived dispersive bands near the Fermi level. Comparison of energy- and angle-dependent data with band-structure calculations reveals renormalization of the 3d bands, but no lower Hubbard band, a signature of the rather weak electron-electron correlations in VS2. High-resolution temperature-dependent low-energy ARPES measurements show the opening of an energy gap at the Fermi level that is attributed to the condensation of the CDW phase. The results indicate a CDW transition in the absence of nesting for 1 T-VS2 © 2010 The American Physical Society.

DOI： 10.1103/PhysRevB.82.075130

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

Electronic structures of the quantum critical superconductor β-YbAlB4 and its polymorph α-YbAlB4 are investigated by using bulk-sensitive hard x-ray photoemission spectroscopy. From the Yb 3d core level spectra, the values of the Yb valence are estimated to be ∼2.73 and ∼2.75 for α- and β-YbAlB4, respectively, thus providing clear evidence for valence fluctuations. The valence band spectra of these compounds also show Yb2+ peaks at the Fermi level. These observations establish an unambiguous case of a strong mixed valence at quantum criticality for the first time among heavy fermion systems, calling for a novel scheme for a quantum critical model beyond the conventional Doniach picture in β-YbAlB4.

DOI： 10.1103/PhysRevLett.104.247201

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

We investigate the electronic structure of chromium nitride (CrN) across the first-order magneto-structural transition at T-N similar to 286 K. Resonant photoemission spectroscopy (PES) shows a gap in the 3d partial density of states at the Fermi level and an on-site Coulomb energy U similar to 4.5 eV, indicating strong electron-electron correlations. Bulk-sensitive high-resolution (6 meV) laser PES reveals a clear Fermi edge indicating an antiferromagnetic metal below T-N. Hard x-ray Cr 2p core-level PES shows T-dependent changes across T-N which originate from screening due to coherent states as substantiated by cluster model calculations using the experimentally observed U. Electrical resistivity confirms an insulator above T-N (E-g similar to 70 meV) becoming a disordered metal below T-N. Thus, CrN transforms from a correlated insulator to an antiferromagnetic metal, coupled to the magnetostructural transition.

DOI： 10.1103/PhysRevLett.104.236404

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

The Magneli phase Ti4O7 exhibits two sharp jumps in resistivity with coupled structural transitions as a function of temperature at T-c1 similar to 142 K and T-c2 = 154 K. We have studied electronic structure changes across the two transitions using 7 eV laser, soft x-ray, and hard x-ray (HX) photoemission spectroscopy (PES). Ti 2p - 3d resonant PES and HX PES show a clear metallic Fermi edge and mixed valency above T-c2. The low temperature phase below T-c1 shows a clear insulating gap of similar to 100 meV. The intermediate phase between T-c1 and T-c2 indicates a pseudogap coexisting with remnant coherent states. HX PES and complementary calculations have confirmed the coherent screening in the strongly correlated intermediate phase. The results suggest the existence of a highly anomalous state sandwiched between the mixed-valent Fermi liquid and charge ordered Mott-insulating phase in Ti4O7.

DOI： 10.1103/PhysRevLett.104.106401

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

We report oil the soft X-ray photoemission and absorption spectroscopy of Ag2NiO2 and Ag2MnO2, which consist of' alternate stacks of (Ag-2)(+) and (MO2)(-) layers. The core-level photoemission and absorption spectra indicate mainly trivalent Ni3+ in Ag2NiO2, and trivalent Mn3+ With an admixture of Mn2+ states in Ag2MnO2. The M 2p-3d resonant photoemission clearly shows that the density of states at the Fermi level includes the M 3d character in both compounds. The results indicate that the Ni 3d and Mn 3d states at the Fermi level are responsible for the significant mass enhancement of carriers, as seen in the transport properties of Ag2NiO2 and Ag2MnO2.

DOI： 10.1143/JPSJ.79.023704

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

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 CeOs 4Sb12 and CeFe4P12 to study their electronic structure and the evolution of states at the Fermi level as a function of temperature (∼10-300K). The Ce3d core level spectra show the presence of f0, f1 and f2 final states with very different relative intensities in the two compounds. Single-impurity Anderson model calculations provide f electron counts of nf = 0.97 and 0.86 per Ce atom, suggestive of a low-and high-TK (= single ion Kondo temperature) for CeOs4Sb12 and CeFe 4P12, respectively. The high-resolution temperature-dependent near-Fermi level spectra show pseudogaps of energy ∼ 50meV and ∼ 110meV in the valence band density of states(DOS) of CeOs 4Sb12 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 CeOs 4Sb12 and CeFe4P12. A compilation of measured pseudogaps using photoemission and optical spectroscopy identifies the charge gaps ΔC for Ce-based Kondo semiconductors and provides a direct relation with TK given by ΔC ∼ 2k BTK. In conjunction with the known behaviour of the spin gaps ΔS ∼ kBTK, the results establish the coupled energy scaling of the spin and charge gaps in Kondo semiconductors. © 2010 IOP Publishing Ltd.

DOI： 10.1088/0953-8984/22/9/095502

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

We have studied the IR reflectance of correlated electron materials under high pressure to 20 GPa, using synchrotron radiation as a highly bright IR source. The instrumentation and examples of obtained results are briefly described.

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

The highly bright infrared (IR) synchrotron radiation from SPring-8 offers various research opportunities, including high pressure IR studies of correlated electron materials using a diamond anvil cell. We present our recent results on the reflectivity studies of an f electron compound CeRu4Sb12, at high pressure up to 8 GPa and at low temperature down to 40 K, measured in both far- and mid-IR ranges.

DOI： 10.1063/1.3326354

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

Infrared spectroscopic studies of various rare-earth compounds (f electron systems) have been made under high pressure, in order to probe their interesting electronic structures induced by pressure. High pressures have been generated with the use of diamond anvil cells (DAC's). To precisely perform IR reflectivity measurements of single crystal samples mounted in a DAC, the synchrotron radiation at SPring-8 has been used as a highly bright IR source. In this manuscript, the basic concept of the high pressure IR experiment using synchrotron radiation is introduced, and an example of obtained data for YbS and their physical interpretation are briefly discussed.

DOI： 10.1088/1742-6596/215/1/012051

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

Optical conductivity [σ(ω)] of YbS has been measured under pressure up to 20 GPa. Below 8 GPa, σ(ω) is low since YbS is an insulator with an energy gap between a fully occupied 4f state and an unoccupied conduction (c) band. Above 8 GPa, however, σ(ω) increases dramatically, developing a Drude component due to heavy carriers and characteristic infrared peaks. It is shown that increasing pressure has caused an energy overlap and hybridization between the c band and 4f state, thus driving the initially ionic and insulating YbS into a correlated metal with heavy carriers. © 2009 The American Physical Society.

DOI： 10.1103/PhysRevLett.103.237202

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

Soft-x-ray photoemission and absorption spectroscopies are employed to investigate the electronic structures of Sr1-x Rh2 O4. Similar to the layered cobaltates such as Na1-x CoO2, a valence-band satellite feature (VBS) occurs at higher binding energy to the O2p band. We find that the VBS resonates at the O1s edge. Additionally, core absorption shows clear x dependence in the O1s edge rather than in the Rh3p edge. These results indicate that the holes in the initial state mainly have O2p character presumably due to d-p rehybridizations affected by Sr2+ vacancy potentials. The resultant inhomogeneous charge texture may have impact on the thermoelectric transport properties at low x. © 2009 The American Physical Society.

DOI： 10.1103/PhysRevB.80.081103

Publishing type：Research paper (scientific journal)  

DOI： 10.1103/PhysRevB.80.081103

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

Electronic structures of La2NiMnO6 epitaxial films are characterized using synchrotron-radiation photoelectron spectroscopy and optical spectroscopy. X-ray absorption spectra reveal that the valence states of Ni2+ and Mn4+ are dominant. The electronic structure at the valence band maximum is mainly derived from the Mn 3d state. The conduction band minimum is composed mostly of the Mn 3d-O 2p hybridized state. The optical gap is estimated to be about 1.5 eV based on the optical conductivity derived from optical spectra.

DOI： 10.1063/1.3159826

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

We have studied the valence redistribution of V in LaAlO3/LaVO3/LaAlO3 trilayers, which are composed of only polar layers grown on SrTiO3 (001) substrates, by core-level photoemission spectroscopy. We have found that the V valence is intermediate between V3+ and V4+ for thin LaAlO3 cap layers, decreases with increasing cap-layer thickness, and finally recovers the bulk value of V3+ at similar to 10 unit-cell thickness. In order to interpret these results, we propose that the atomic reconstruction of the polar LaAlO3 surface competes with the purely electronic V valence change so that the polar catastrophe is avoided at the cost of minimum energy.

DOI： 10.1103/PhysRevLett.102.236401

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

We study epitaxial NdNiO3 thin films on NdGaO3(001) single-crystal substrates grown using a pulsed-laser deposition method. The films show a clear first-order metal-insulator transition (MIT) at T-MI similar to 240 K, which is significantly higher than T-MI similar to 190 K in bulk NdNiO3. The x-ray reciprocal space map shows in-plane tensile and out-of-plane compressive strain, stabilizing the more distorted NdNiO3 thin films with the higher transition temperature. Hard x-ray photoemission shows changes across the MIT and the bandwidth-controlled charge-transfer gap opens due to the reduced p-d hybridization in the low-temperature insulating phase.

DOI： 10.1063/1.3086666

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

We report the three-dimensional (3D) momentum-resolved soft x-ray photoemission spectroscopy of the Fermi liquid LaNiO3. The out-of-plane and in-plane cuts of the 3D electron- and hole-Fermi surfaces (FSs) are observed by energy- and angle-dependent photoemission measurements. The energy bands forming the electron FS suggest an omega(2) dependence of the imaginary part of the self-energy and a "correlation kink" at an energy scale of 0.25 eV. In contrast, the bands which form nesting character hole FSs do not show kinks and match local-density approximation calculations. The results indicate a momentum-dependent mass renormalization, leading to electron-hole asymmetry in strongly correlated LaNiO3.

DOI： 10.1103/PhysRevB.79.115122

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

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

We study the electronic structure of the filled skutterudite CeOs4Sb12 using photoemission spectroscopy (PES). Soft x-ray excited Ce 3d-4f resonant PES confirms the existence of Ce 4f states at the Fermi level (EF). Temperature dependent high-resolution laser-PES spectra reveal a pseudogap formation around EF, which can be explained in terms of the hybridization gap. Simultaneously, a sharp feature is formed just above EF with decreasing temperature. The heavy-fermion-like specific heat is attributed to the occupation of this feature. The results identify the origin of the anomalous coexistence of heavy-fermion and pseudogap behavior in terms of a symmetry dependent hybridization.

DOI： 10.1103/PhysRevLett.102.036403

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

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

Hard x-ray photoemission and optical spectroscopies have been performed on YbS and Yb metal to determine the precise f-electron occupation. A comparison of the photoemission spectra with the energy-loss functions in bulk and surface, obtained from optical reflectivity, enables us to distinguish between the energy-loss satellite of Yb2+ peak and the Yb3+ multiplet. The results clearly indicate a purely divalent Yb state except for the surface of YbS which is altered by oxidation. We demonstrate that the present method is highly reliable in identifying the electronic structure and the mean valence in f-electron systems.

DOI： 10.1103/PhysRevB.78.195118

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

A low power density very-high-frequency (VHF) (55 MHz) H-2 plasma in a capacitively coupled multihole-cathode (MHC) geometry is studied using Langmuir probe measurements. Radial profiles show a higher ion density (N-i) and lower electron temperature (T-e) compared to a MHC 13.56 MHz H-2 plasma. The N-i dependence on power indicates an Ohmic plasma, while T-e is essentially constant. The MHC-VHF plasma is used to investigate mixed phase microcrystalline+amorphous (mu c+a-) Si:H thin films at a substrate temperature of 60 degrees C. High-resolution photoemission suggests two types of Si, with concentrations in agreement with atomic force microscopy images showing similar to 510 +/- 40 nm crystallites embedded in a-Si:H matrix. The results show that the low power density MHC-VHF plasma is a high-N-i Ohmic collisional plasma, suitable for low temperature deposition of mu c+a-Si:H thin films.

DOI： 10.1063/1.3023066

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

The temperature (T)-dependent metal-insulator transition (MIT) in VO(2) is investigated using bulk sensitive hard-x-ray (similar to 8 keV) valence-band, core-level, and V 2p-3d resonant photoemission spectroscopies (PESs). The valence-band and core-level spectra are compared with full-multiplet cluster model calculations including a coherent screening channel. Across the MIT, V 3d spectral weight transfer from the coherent (3d(1)(C) under bar final) states at Fermi level to the incoherent (3d(0)+3d(1)(L) under bar final) states, corresponding to the lower Hubbard band, leads to gap formation. The spectral shape changes in V 1s and V 2p core levels as well as the valence band are nicely reproduced from cluster model calculations, providing electronic structure parameters. Resonant PES finds that the 3d(1)(L) under bar states resonate across the V 2p-3d threshold in addition to the 3d(0) and 3d(1)(C) under bar states. The results support a Mott-Hubbard transition picture for the first-order MIT in VO(2).

DOI： 10.1103/PhysRevB.78.075115

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

Soft x-ray photoelectron spectroscopy for FeSi1-xGex (x=0, 0.11, 0.19, 0.26, and 0.44) and FeGa3 was performed at Fe L absorption edge complementary to the x-ray emission spectroscopy. The absorption and valence-band spectra show the phase transition of the electronic state between x=0.26 and 0.44 in FeSi1-xGex accompanying the narrowing of the density of state near the Fermi edge. High-resolution x-ray absorption spectra (total electron yields) at Fe 2p(3/2) absorption edge resolve the fine structure, originated from the valence and Auger component of electrons. Resonant valence-band and constant initial-state spectra show Fano-type profile at Fe 2p(3/2) absorption edge only for the x=0.44 sample but not for the x=0 sample and FeGa3. The crossover point, where the Raman and Auger features mix, is near the Fe 2p absorption edge for both of the x=0 and 0.44 samples, showing the delocalized 3d states.

DOI： 10.1103/PhysRevB.78.045125

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

We have reexamined the valence-band (VB) and core-level electronic structure of NiO by means of hard and soft x-ray photoemission spectroscopies. The spectral weight of the lowest energy state was found to be enhanced in the bulk sensitive Ni 2p core-level spectrum. A configuration-interaction model including a bound state screening has shown agreement with the core-level spectrum and off- and on-resonance VB spectra. These results identify the lowest energy states in the core-level and VB spectra as the Zhang-Rice (ZR) doublet bound states, consistent with the spin-fermion model and recent ab initio calculations within dynamical mean-field theory. The results indicate that the ZR character first ionization (the lowest hole-addition) states are responsible for transport properties in NiO and doped NiO.

DOI： 10.1103/PhysRevLett.100.206401

Publishing type：Research paper (scientific journal)  

Optical conductivity spectra [σ (ω)] have been measured for many heavy-fermion (HF) Ce and Yb compounds. A characteristic mid-infrared peak has been observed in σ (ω) of these compounds, and has been analyzed in terms of the conduction (c)-f electron hybridized bands. A universal scaling is found in terms of the observed peak energies for these HF compounds. This optical scaling is discussed in comparison with other universal relations found for HF compounds such as the Kadowaki-Woods relation. © 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2007.10.027

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

Semiconducting skutterudite CeFe(4)P(12) is investigated by synchrotron x-ray photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS). Ce 3d core-level PES and 3d-4f XAS, in combination with single-impurity Anderson model (SIAM) calculations, confirm features due to f(0), f(1), and f(2) configurations. The Ce 3d-4f resonant-PES spectra provide the Ce 4f density of states (DOS), which indicates the absence of a Kondo resonance at the Fermi level, but can still be explained by SIAM with a small gap in non-f DOS. While Ce 4f partial DOS from band structure calculations is consistent with the main Ce 4f DOS, the importance of SIAM for core-level PES and XAS spectra quantifies the mixed valence for semiconducting CeFe(4)P(12), derived from strong hybridization between non-f conduction and Ce 4f DOS.

DOI： 10.1103/PhysRevB.77.165126

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

The infrared reflectivity spectra under pressure have been measured for filled-skutterudite PrFe4P12. In the far-infrared (FIR) region, four phonon peaks became prominent and shifted monotonically for higher energy side under pressure. The pressure induced insulator phase at low temperature was stable up to 18 GPa without any large structural change through the metal-insulator transition. (c) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2007.10.241

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

Resonant photoemission at the Ti 2p and O 1s edges on a Nb-doped SrTiO3 thin film revealed that the coherent state (CS) at the Fermi level (E-F) had a mainly Ti 3d character whereas the incoherent in-gap state (IGS) positioned similar to 1.5 eV below E-F had a mixed character of Ti 3d and O 2p states. This indicates that the IGS is formed by a spectral-weight transfer from the CS and subsequent spectral-weight redistribution through d-p hybridization. We discuss the evolution of the excitation spectrum with 3d band filling and rationalize the IGS through a mechanism similar to that proposed by Haldane and Anderson.

DOI： 10.1103/PhysRevLett.100.056401

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

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

The existence of an energy gap around the Fermi energy is established experimentally for the intermetallic compound FeGa3. Fe-57 Mossbauer spectra measured at room temperature confirmed the absence of magnetic ordering. The temperature dependence of magnetic susceptibility, chi(T), measured on single-crystalline samples showed diamagnetic behavior below room temperature, and it increased exponentially with temperature above 500 K. Fitting to the high-temperature data yielded an energy gap A of 0.29-0.45 eV. Valence-band x-ray photoemission spectra directly revealed the energy gap, the size of which was estimated to be Delta < (similar to)under tilde> 0.8 eV. These results agree with the prediction by recent band calculations.

DOI： 10.1143/JPSJ.77.024705

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

We have studied the electronic structure of multilayers composed of a band insulator LaAlO(3) (LAO) and a Mott insulator LaVO(3) (LVO) by means of hard x-ray photoemission spectroscopy, which has a probing depth as large as similar to 60 A. The Mott-Hubbard gap of LVO remained open at the interface, indicating that the interface is insulating unlike the LaTiO(3)/SrTiO(3) multilayers. We found that the valence of V in LVO was partially converted from V(3+) to V(4+) only at the interface on the top side of the LVO layer and that the amount of V(4+) increased with LVO layer thickness. We suggest that the electronic reconstruction to eliminate the polarity catastrophe inherent in the polar heterostructure is the origin of the highly asymmetric valence change at the LAO/LVO interfaces.

DOI： 10.1103/PhysRevB.77.045122

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

The pressure-induced metal-insulator transition in PrRu4P12 has been examined by the optical study at 20 K. The reflectivity and the optical conductivity clearly showed the change from an insulator to a metal across 8 GPa. The temperature-induced gap was stabilized by pressure in the realm of insulating state below 8 GPa. The typical metallic spectra with a finite Drude component were observed in both reflectivity and optical conductivity above 8 GPa.

DOI： 10.1143/JPSJS.77SA.214

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

Optical conductivity [sigma (omega)] spectra have been measured for eight Ce-filled skutterudite compounds CeT4X12 (T=Fe, Ru, Os; X=P, As, Sb) to systematically study their microscopic electronic structures around Fermi level. The sigma(omega) spectra show large differences among X=P, As and Sb, while they show smaller differences among T=Fe, Ru and Os for a given X element. This indicates that the choice of X has larger effects on the electronic structures of CeT4X12 than that of T. The spectral shape and temperature dependence of sigma (omega) suggest that the electronic structures of CeT4P12 are similar to that of a band semiconductor and those of CeT4Sb12 to that of a heavy fermion metal or a Kondo semiconductor. In the case of CeT4As12, the sigma(omega) spectra suggest intermediate characteristics between CeT4P12 and CeT4Sb12, namely they are similar to those shown by a semiconductor with a very narrow energy gap.

DOI： 10.1143/JPSJS.77SA.315

Publishing type：Research paper (scientific journal)  

DOI： 10.1103/PhysRevLett.101.137601

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

We investigate the electronic structure of 3d(1) configuration vanadium oxides (VO2, SrVO3 and CaVO3) using soft X-ray (SX) and hard X-ray (HX) photoemission spectroscopy (PES). The valence band spectra of VO2 in SX- and HX-PES clearly show the opening of a gap at E-F across the metal-insulator transition. Spectral changes in the V 2p and O 1s core levels were also observed. In addition, 3d(1) configuration correlated metals SrVO3 and CaVO3 are typical materials whose valence band spectra are compared with dynamical mean-field theory. The V 2p and V 1s core level HX-PES of SrVO3 and CaVO3 show clear additional well-screened features. These features originate from bulk screening by a coherent band at EF. These spectra are nicely reproduced by a cluster model calculation, indicating a difference of the parameter V* for the interaction strength between the central V 3d orbitals and the coherent band. (c) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.elspec.2006.12.055

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

We study the electronic structure of Mott-Hubbard systems SrVO3 and CaVO3 with photoemission spectroscopy (PES) using hard Xrays (HX) (hv similar to 8 keV) and soft X-rays (SX) (hv similar to 900 eV). In HX-PES results, the V 2p core levels of SrVO3 and CaVO3 show clear additional intensity, which are not observed in SX-PES. These features originate from bulk screening by coherent states at Fermi level as observed in other 3d transition metal oxides. From the comparison between SrVO3 and CaVO3 in HX-PES, the intensity in SrVO3 is nearly similar to CaVO3. The results suggest screening from coherent states is more efficient in HX-PES compared to SX-PES. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmmm.2006.10.207

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

The optical study has been performed on filled-skutterudite PrFe4P12 applying pressure up to 16 GPa. The reflectivity at far-infrared (FIR) region showed that the metallic reflectivity looses its intensity and the weak phonon peaks at ambient pressure become prominent with pressures at lower temperature. It insists that the electronic states near Fermi level in this compound changes drastically from metallic properties to insulating ones at high pressures and low temperatures, and the insulating phase persists up to 16 GPa against the electrical resistivity data under pressure. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmmm.2006.10.009

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

Dynamical conductivity spectra [sigma(omega)] have been measured for many heavy-fermion (HF) Cc and Yb compounds. A characteristic excitation peak has been observed in the infrared region of sigma(omega) for all the compounds, and has been analyzed in terms of a simple model based on conduction (c)-f electron hybridized band. A universal scaling is found between the observed peak energies and the estimated c-f hybridization strengths of these HF compounds. This scaling demonstrates that the model of c-f hybridized band can generally and quantitatively describe the low-energy charge excitations in a wide range of HF compounds.

DOI： 10.1143/JPSJ.76.023703

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

It has been known that the elemental Yb, a divalent metal at ambient pressure, becomes a mixed-valent metal under external pressure, with its valence reaching similar to 2.6 at 30 GPa. In this work, infrared spectroscopy has been used to probe the evolution of microscopic electronic states associated with the valence crossover in Yb at external pressures up to 18 GPa. The measured infrared reflectivity spectrum [R(omega)] of Yb has shown large variations with pressure. In particular, R(omega) develops a deep minimum in the mid-infrared, which shifts to lower energy with increasing pressure. The dip is attributed to optical absorption due to a conduction (c)-f electron hybridization state, similarly to those previously observed for heavy fermion compounds. The red shift of the dip indicates that the c-f hybridization decreases with pressure, which is consistent with the increase of valence.

DOI： 10.1143/JPSJS.76SA.9

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

We have constructed a high-resolution synchrotron-radiation photoemission apparatus combined with a laser molecular-beam epitaxy (laser MBE) system in order to investigate the electronic structure of thin films. The system is installed at the newly-built soft x-ray undulator beamline BL17SU of SPring-8. Single crystal thin films fabricated by laser MBE can be transferred quickly into the photoemission chamber under ultra-high vacuum condition. The photoemission spectrometer is equipped for high-throughput and high energy-resolution angle-resolved photoemission measurements. High energy-resolution and stability of the beamline optics enable us to achieve the best total energy resolution of 51 meV at 867 eV excitation. The performance and characteristics of the system is demonstrated by showing results on LaNiO 3 thin films. © 2007 American Institute of Physics.

DOI： 10.1063/1.2436374

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

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

DOI： 10.1143/JPSJS.76SA.13

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

We investigate the temperature induced mixed valence transition in ELiNi2(Si0.2Ge0.8)(2) using Eu 3d-4f X-ray absorption spectroscopy (XAS). The unoccupied 4f states are studied as a function of temperature (27-120 K). across the critical valence temperature, T-v = 80 K. The Eu 3d-4f X-ray absorption spectra show systematic changes of the mixed valency with temperature. The Eu2+ and Eu3+ spectral features match very well with atomic multiplet calculations. The observed non-integral mean valence changes systematically from similar to 2.70 +/- 0.03 (27 K) to similar to 2.35 +/- 0.03 (120 K), consistent with bulk-sensitive measurements. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2006.01.507

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

We have investigated the electronic structure of filled skutterudites CeFe4P12 and CeOs4Sb12 by means of Cc 3d-4f resonant photoemission spectroscopy. The difference in the spectral property of on-resonant photoemission spectra can be explained by the degree of hybridization between conduction band states and Cc 4f states. In addition, the spectral intensity at the Fermi level for CeFe4P12 is significantly lower than that for CeOs4Sb12. The results indicate that a Kondo resonance exists in CeOs4Sb12 while Kondo resonance is suppressed in CeFe4P12. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2006.01.068

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

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

FeS is one of the 3d transition-metal compounds and undergoes a pressure-induced metal-insulator transition accompanied by the successive structural phase transition from a troilite (semiconductor) to a hexagonal structure (metallic) at around 3.5 GPa and finally to a monoclinic structure at 6.6 GPa at room temperature. In this paper, we report the recent results of the spectromicroscopic study on Fes under pressure in the far-infrared region to observe the change in the electronic states. (c) 2006 Published by Elsevier B.V.

DOI： 10.1016/j.physb.2006.01.532

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

DOI： 10.11316/jpsgaiyo.61.1.3.0_582_4

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

DOI： 10.11316/jpsgaiyo.61.1.3.0_601_1

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

We study the mixed valence transition (Tv∼80K) in EuNi2(Si0.2Ge0.8)2 using Eu 3d-4f x-ray absorption spectroscopy (XAS) and resonant photoemission spectroscopy (RESPES). The Eu2+ and Eu3+ main peaks show a giant resonance and the spectral features match very well with atomic multiplet calculations. The spectra show dramatic temperature (T) dependent changes over large energies (∼10eV) in RESPES and XAS. The observed nonintegral mean valencies of ∼2.35±0.03(T=120K) and ∼2.70±0.03(T=40K) indicate homogeneous mixed valence above and below Tv. The redistribution between Eu2+4f7+[spd]0 and Eu3+4f6+[spd]1 states is attributed to a hybridization change coupled to a Kondo-like volume collapse. © 2005 The American Physical Society.

DOI： 10.1103/PhysRevB.72.161101

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

We study the mixed valence transition (T-v similar to 80 K) in EuNi2(Si0.2Ge0.8)(2) using Eu 3d-4f x-ray absorption spectroscopy (XAS) and resonant photoemission spectroscopy (RESPES). The Eu2+ and Eu3+ main peaks show a giant resonance and the spectral features match very well with atomic multiplet calculations. The spectra show dramatic temperature (T) dependent changes over large energies (similar to 10 eV) in RESPES and XAS. The observed nonintegral mean valencies of similar to 2.35 +/- 0.03 (T=120 K) and similar to 2.70 +/- 0.03 (T=40 K) indicate homogeneous mixed valence above and below T-v. The redistribution between Eu(2+)4f(7)+[spd](0) and Eu(3+)4f(6)+[spd](1) states is attributed to a hybridization change coupled to a Kondo-like volume collapse.

DOI： 10.1103/PhysRevB.72.161101

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

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

DOI： 10.1103/PhysRevB.72.073105

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

The spinel compound CuIr2Se4 at atmospheric pressure is a metal of wide temperature range, but undergoes a stabilization of the insulating phase from the metal phase at high pressures above 3 GPa. To study the change in electronic state at pressure, we performed a spectromicroscopic measurement in the far-infrared to visible region on polycrystalline CuIr2Se4 at high pressures up to 8 GPa at room temperature. The peak structure at around 0.2 eV in the optical conductivity spectra at atmospheric pressure, whose origin was assigned to the hybridized band, was found to lose its intensity at pressure accompanied by the rapid suppression of the Drude component. This change in the optical spectra at pressure indicates that the pseudo-gap state formed in the vicinity of the Fermi level is very sensitive to pressure. We discuss this behaviour in relation to the instability of the metallic state due to the increase in the degree of hybridization with pressure.

DOI： 10.1143/JPSJ.74.1099

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

DOI： 10.11316/jpsgaiyo.60.2.3.0_397_3

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：American Institute of Physics Inc.  

We have designed a new infrared beam line BL6B at UVSOR for infrared and terahertz spectroscopies including microspectroscopy with high brilliance and high flux. The beam line will be replaced in the spring of 2004 from the infrared beam line, BL6A1, at UVSOR. The beam line has a large acceptance angle of 215(H) × 80(V) mrad2 and a so-called "magic mirror" is adopted to get the perfect focusing of the bending magnet radiation. The optics and expected performance (focus size, photon flux and brilliance) are reported.

DOI： 10.1063/1.1757822

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

Infrared study has been made on the filled skutterudites PrRu4P12 and SmRu4P12; which undergo a metal-insulator (M-I) transition at T-MI = 60 and 16 K; respectively. The opening of a charge gap for the two compounds are clearly observed below TMI: For PrRu4P12; new phonon peaks are also observed below T-MI, which suggests that the crystal symmetry is lowered. On the other hand, no additional peaks appear in the insulating phase for SmRu4P12: These results indicate that the mechanism of M-I transition of SmRu4P12 is different from that of PrRu4P12: (C) 2003 Elsevier B. V. All rights reserved.

DOI： 10.1016/j.jmmm.2003.11.134

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

We have measured the optical conductivity sigma(1)(omega) of the filled skutterudites CeFe4Sb12; CeRu4Sb12; and CeOs4Sb12: These sigma(1)(omega) spectra are explained in terms of the hybridization between Ce 4f states and conduction (c) electron states. From the comparison of the sigma(1)(omega) spectra, it is concluded that the degree of c-f hybridization is strongest for CeFe4Sb12 and weakest for CeOs4Sb12: (C) 2003 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmmm.2003.11.137

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

DOI： 10.11316/jpsgaiyo.59.1.3.0_609_4

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

DOI： 10.11316/jpsgaiyo.59.2.3.0_476_3

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

DOI： 10.11316/jpsgaiyo.59.1.3.0_570_3

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

CuIr2Se4 is a metal at atmospheric pressure, but shows a stabilization of insulating phase from metal phase under high pressure. In order to study the electronic structure very near to the Fermi Level (E-F) and its change due to a metal-insulator transition (MIT), we measured the temperature and pressure dependence of optical spectra on polycrystalline spinel compound CuIr2Se4 under high pressure. At ambient pressure, the growth of the peak structure at similar to0.2 eV corresponding to the formation of a pseudo-gap state was found with decrease in temperature but a Drude component still remained even at 8 K. Under pressure, the a-spectra changed much from that at ambient pressure. At 0.5 GPa, two peaks at similar to0.2 eV and at similar to0.8 eV corresponding to interband transitions were found together with a Drude component. With pressure, the peak at similar to0.2 eV that were assigned to the electronic excitation across the hybridization bands of Ir5d and Se4p states lost its intensity as the Drude component did at 5.5 GPa. At 6.0 GPa, the spectrum showed the disappearance of the Drude component and the shift to high energy of two interband transition peaks. Our experiments indicated clearly the pressure-induced MIT due to the increase in the degree of hybridization.

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

We have studied the optical conductivity of the filled skutterudite CeOs4Sb12. The optical conductivity spectra show a strong temperature dependence and reveal an energy gap of $\sim$30 meV below 60 K. This shows that the density of states near the Fermi level of CeOs4Sb12 is strongly reduced. In addition to the gap formation, a pronounced peak is observed at $\sim$70 meV below 160 K. This mid-infrared peak is interpreted in terms of optical excitations across a hybridization gap consisting of conduction- and $f$-electron states. The observed characteristics in the optical spectra are quite similar to those previously reported for Kondo semiconductors. We compare the optical characteristics of CeOs4Sb12 found in the present work with other physical properties, and analyze its electronic structures.

DOI： 10.1143/JPSJ.72.2722

Other Link： https://jlc.jst.go.jp/DN/JALC/00233762577?from=CiNii

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

DOI： 10.11316/jpsgaiyo.58.2.3.0_546_3

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

DOI： 10.11316/jpsgaiyo.58.2.3.0_531_3

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

DOI： 10.11316/jpsgaiyo.58.2.4.0_669_4

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

The optical reflectivity spectra of the Kondo semiconductors CeNiSn. CeRhSb, and CeRhAs have been measured in a wide photon energy range to obtain the optical conductivity spectra sigma(omega). For CeNiSn and CeRhSb, two pronounced peaks in the mid-infrared (IR) region and small pseudogaps in the far-1R region were observed at low temperatures T). The two peaks are attributed to optical transitions related with the two hands arising from the spin-orbit splitting of Ce 4f, For CeRhAs, sigma(omega) exhibits a gap which develops gradually with cooling. The observed (pseudo)gap widths and T dependent variations are largest for CeRhAs and smallest for CeNiSn, well corresponding to their K.ond.o temperatures. For CeNiSn and CeRhSb, the T dependence of sigma(omega) in the far-1R suggests the formation of a narrow Drude peak caused by a f mixing. The results may indicate that the pseudogap is opened in a heavy quasiparticle hand.

DOI： 10.1143/JPSJS.71S.291

Publishing type：Research paper (scientific journal)  

Optical conductivity data [σ(ω)] of the colossal magnetoresistance (CMR) pyrochlore Tl2Mn2O7 are presented as functions of temperature (T) and external magnetic field (B). Upon cooling and upon applying B near the Curie temperature, where the CMR manifests itself, σ(ω) shows a clear transition from an insulatorlike to a metallic electronic structure as evidenced by the emergence of a pronounced Drude-like component below ∼0.2 eV. Analyses on the σ(ω) spectra show that both T- and B-induced evolutions of the electronic structure are very similar to each other, and that they are universally related to the development of macroscopic magnetization (M). In particular, the effective carrier density obtained from σ(ω) scales with M2 over wide ranges of T and B. The contributions to the CMR from the carrier effective mass and scattering time are also evaluated from the data.

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

Optical conductivity data [sigma(omega)] of the colossal magnetoresistance (CMR) pyrochlore Tl2Mn2O7 are presented as functions of temperature (T) and external magnetic field (B). Upon cooling and upon applying B near the Curie temperature, where the CMR manifests itself, sigma(omega) shows a clear transition from an insulatorlike to a metallic electronic structure as evidenced by the emergence of a pronounced Drude-like component below similar to0.2 eV. Analyses on the sigma(omega) spectra show that both T- and B-induced evolutions of the electronic structure are very similar to each other. and that they are universally related to the development of macroscopic magnetization (M). In particular, the effective carrier density obtained from sigma(omega) scales with M-2 over wide ranges of T and B. The contributions to the CMR from the carrier effective mass and scattering time are also evaluated from the data.

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

Optical conductivity data (formula presented) of the colossal magnetoresistance (CMR) pyrochlore (formula presented) are presented as functions of temperature (T) and external magnetic field (formula presented) Upon cooling and upon applying B near the Curie temperature, where the CMR manifests itself, (formula presented) shows a clear transition from an insulatorlike to a metallic electronic structure as evidenced by the emergence of a pronounced Drude-like component below (formula presented) Analyses on the (formula presented) spectra show that both T- and B-induced evolutions of the electronic structure are very similar to each other, and that they are universally related to the development of macroscopic magnetization (formula presented) In particular, the effective carrier density obtained from (formula presented) scales with (formula presented) over wide ranges of T and B. The contributions to the CMR from the carrier effective mass and scattering time are also evaluated from the data. © 2001 The American Physical Society.

DOI： 10.1103/PhysRevB.64.180409

Language：English   Publishing type：Rapid communication, short report, research note, etc. (scientific journal)   Publisher：Physical Society of Japan  

Since the discovery of unconventional quantum criticality in the Au–Al–Yb quasicrystal and pressure-induced quantum criticality in its approximant, quasicrystals and approximants with an icosahedral cluster of atoms have attracted much attention. Here we report magnetic, thermodynamic, transport, and hard x-ray photoemission spectroscopy experiments on the Ce-based approximant Ag–In–Ce. These results indicate the heavy fermion behavior of the 4 f electrons and provide convincing evidence of a magnetic phase transition into a spin-glass-like short-range-ordered state at a low temperature. The Ag–In–Ce alloy is the first Ce-based heavy fermion approximant to a Tsai-type quasicrystal. Reflecting its unique crystal structure, the approximant shows an electrical resistivity distinct from that of traditional Kondo lattices.

DOI： 10.7566/JPSJ.86.093702

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

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

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

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

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

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

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

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

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

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

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

DOI： 10.11316/jpsgaiyo.66.1.3.0_582_3

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

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

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

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

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

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

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

Language：Japanese   Publisher：The Physical Society of Japan  

DOI： 10.11316/jpsgaiyo.65.2.3.0_531_3

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

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

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

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

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

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

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

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

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

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

DOI： 10.11316/jpsgaiyo.64.1.3.0_652_1

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

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

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

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

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

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

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

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

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

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

Language：English   Publishing type：Research paper, summary (international conference)   Publisher：ELSEVIER SCIENCE BV  

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

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

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

Language：English   Publisher：AMER INST PHYSICS  

Recent electrical resistivity measurements under high pressure revealed that materials with strongly correlated electrons exhibit many interesting physical properties such as a stabilization of insulating phase, generation of unconventional superconductivity, and so on. However, there seems to be no attempt by optical measurements to study directly the change in the electronic states from metallic phase to insulating one under high pressure in GPa range because of the difficulty in experiments in the far-infrared region. The difficulty is mainly caused by the insufficiency in the brightness of conventional black body sources. SPring-8 supplies a more collimated and brilliant infrared beam to the end station than conventional thermal sources, and then make possible a spectromicroscopic measurement under high pressures above 10 GPa in the far-infrared region. In this paper, we report recent results of the optical study on several kinds of solids which exhibit a cross-over change in the electronic states under high pressure.

Language：English   Publisher：AMER INST PHYSICS  

Photoemission Spectroscopy (PES) is a powerful method to investigate electronic structure of materials. However, conventional vacuum ultraviolet (VUV) and soft x-ray (SX) PES is surface sensitive because of the short inelastic-mean-free-paths of photoelectrons. This requires us to prepare clean surface and sometimes obscures intrinsic bulk states. In order to realize bulk-sensitive or surface-insensitive PES, we have developed hard x-ray (HX) PES using high-brilliance synchrotron radiation at BL29XU in SPring-8. Large probing depth of high energy photoelectrons enables us to probe intrinsic bulk states almost free from surface condition. A combination of x-ray optics and an electron energy analyzer dedicated for HX-PES achieved the total instrumental energy resolution of 63 and 55 meV (FWHM) at 5.95 and 7.94 keV, respectively. A special arrangement of an analyzer and a sample was employed and increased photoelectron intensity drastically. We describe present performance of our apparatus and characterize HX-PES by showing typical spectra.

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

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

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

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

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

Language：Japanese  

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

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

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

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

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

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

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

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

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

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

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

Language：English   Publisher：ELSEVIER SCIENCE BV  

We have investigated the temperature dependence of the optical conductivity of GdRu4P12, which shows an antiferromagnetic ordering below T-N = 22 K. Above T-N, the optical conductivity spectra exhibit a Drude-like feature due to the free carriers. However, below T-N a new peak was found to develop around 30 meV in addition to the narrowing of the Drude component. We assigned this peak to the electronic excitation across the pseudo-gap around the Fermi level induced by the antiferromagnetic ordering. The narrowing of the Drude component reflects the growth of the coherence state due to the formation of the antiferromagnetic ordering. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2005.01.247

Language：English   Publisher：ELSEVIER SCIENCE BV  

We have developed infrared spectromicroscopic system which works under high pressure up to 20 GPa by using a diamond-anvil-cell combined with brilliant infrared radiation source from SPring-8, the storage ring facilities at Harima in Japan. This system is available from far-infrared to visible region. As a typical example, the optical reflection spectra of a polycrystalline CuIr2Se4 were measured at pressure up to 8 GPa at room temperature. Clear change in the electronic states under pressure was observed. In the optical conductivity spectra, the electronic states in the vicinity of the Fermi level was found to become unstable and lose its intensity with pressure in addition to the decay of the Drude component by free carriers. We discussed this behavior in relation with the instability of the metallic state due to the increase in the degree of the hybridization with pressure. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2005.01.340

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

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

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

Language：Japanese   Publisher：The Physical Society of Japan  

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

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

Language：English   Publisher：ELSEVIER SCIENCE BV  

We have measured the optical conductivity spectra sigma(omega) of YbB12 and YbAl3; which are typical mixed-valent Yb compounds. The measured sigma(omega) spectra in the mid-infrared region are quite similar for the two compounds, with a pronounced peak centered near 0.2 eV: This result demonstrates that the Kondo semiconductor YbB12 and the highly metallic YbAl3 share very similar electronic structures near the Fermi level, in spite of their very different transport properties at low temperatures. This observation is discussed in terms of c-f hybridization states in these compounds. (C) 2003 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmmm.2003.11.151

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

DOI： 10.11316/jpsgaiyo.59.1.3.0_642_4

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

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

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

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

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

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

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Other Link： http://hdl.handle.net/2433/97489

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

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

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

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

Language：English  

We have studied the evolution of energy gap in the optical conductivity [σ(ω)] of Yb1-xLuxB12 as functions of temperature (T) and non-magnetic Lu substitution (x). YbB12 has a well-developed energy gap in σ(ω) below 20 K, with the onset of σ(ω) at ∼20 meV and a shoulder at ∼40 meV. With increasing T or x, the energy gap is rapidly filled in, which indicates the importance of coherence among the Yb 4f orbitals at different sites. The shoulder, on the other hand, remains nearly unshifted in wide ranges of T (to 70 K @) and x (0-1/2), which shows that the characteristic energy for the gap is given by some single-site energy scale of Yb3+. Namely, both lattice and single-site effects play important roles in the gap formation of YbB12. © 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4526(01)01074-2

Language：English   Publisher：ELSEVIER SCIENCE BV  

Pyrochlore Tl2Mn2O7 has been found to exhibit a colossal magneto-resistance (CMR) that is comparable to that for the perovskite manganites such as La1-xSrxMnO3. In order to probe the microscopic electronic structures of Tl2Mn2O7, we have measured its optical conductivity sigma(omega) in wide ranges of photon energy, temperature (T), and external magnetic field (B). We have observed very strong variations of sigma(omega) with T and B near the Curie temperature, where the CMR manifests itself. Analyses on the variations in sigma(omega) show that both T- and B-induced evolutions of the electronic structures in Tl2Mn2O7 are very similar to each other, and that they are universally related to the development of macroscopic magnetization. We have also observed a pronounced Kerr rotation at the plasma edge of the reflectivity. (C) 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4526(01)01220-0

Language：English   Publisher：PHYSICAL SOC JAPAN  

We present our optical studies on the Kondo semiconductor YbB12, and on the "diluted" compounds Yb1-xLuxB12 (0 <= x <= 1). The optical conductivity spectrum sigma(omega) of YbB12 is characterized by the opening of an energ gap of similar to 20 meV below 80 K, and a pronounced mid-infrared (IR) peak which is also strongly temperature (T) dependent. For Yb1-xLuxB12 with increasing x, the clear gap for x=0 is rapidly filled in from the bottom, and the mid-IR. peak shows large red shifts. It is shown that increasing T and increasing x have qualitatively very similar effects on the energy gap and the mid-IR peaks. We discuss the evolutions of Yb 4f-related electronic states near the Fermi level with T and x, and its relation to the gap evolution.

DOI： 10.1143/JPSJS.71S.303

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

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

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

Language：English   Publisher：Elsevier Ltd  

Language：English   Publisher：Elsevier Ltd  

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

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

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

Language：English  

We have measured the optical conductivity σ(ω) of Yb1-xLuxB12 (0≤x≤1), where the system evolves from a Kondo semiconductor at x=0 to a nonmagnetic metal at x=1. For x=0, σ(ω) exhibits a clear energy gap with an onset at ∼20 meV and a shoulder at 38 meV. As x increases, the gap is rapidly filled in from the bottom, while the shoulder remains at ∼40 meV up to x=1/2. These results suggest that Lu substitution into YbB12 produces an in-gap band, but the characteristic energy for the gap remains unchanged in a wide range of x. Spectral evolutions of a characteristic mid-infrared peak, which results from Yb 4f-derived states near the Fermi level, suggest a strong coupling of Yb 4f electrons with conduction electrons.

DOI： 10.1103/PhysRevB.62.R13265

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

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

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

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

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

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