Published papers

多結晶CoPn2 (Pn = As, Sb)の電気伝導と熱伝導を、300-900 Kの間で報告する。CoAs2は高温まで半導体的な電気抵抗率の温度依存性を示すのに対し、CoSb2はこの温度領域では金属的な縮退した半導体となる。いずれも10 Wm-1K-1の熱電材料としては高い熱伝導率を示すため、熱電変換無次元性能指数ZTは0.1 付近にある。

新規熱電変換材料として期待されるSnAs層をもつEuSn2As2の多結晶合成の詳細を報告する。 固相反応に用いるルツボとして、アルミナとカーボンの二種を検討し、いずれの場合も主相としてEuSn2As2相を得た。カーボンを使用した場合に、自己フラックとしての、Sn以外の異相の検出されない試料を得た。本報告は、2017年のArguillaらによる試料合成につづいて、2例目であり、Arguillaらの試料と格子定数が異なる。

ZrCuSiAs型の層状構造は, p型半導体機能, 高機能熱電変換機能, 及び高温超伝導などの様々な機能性を示す. 本稿ではZrCuSiAs型化合物の内, 鉄系高温超伝導体として知られる LaFeAsO1-xFxに着目し, そのスピン分極の変化を密度汎関数理論により検証した. 実験で観察されるフッ素濃度上昇に伴うスピン分極の消失がよく再現された.

2012年にKitanoらにより室温で安定な電子化物(エレクトライド)[Ca24Al28O64]4+(e-)4 (Ru/C12A7:e-)を担体とし, Ru金属微粒子を担持した構造が, アンモニア合成の高効率触媒として働くことが報告された. われわれは, その機構を密度汎関数理論を用いて, 部分的ながら解析した. スラブモデルを用いて, C12A7:e-上に存在するRu原子層の安定性を検討したところ, Ru原子層一層の場合にのみ, C12A7とRuの双方が固体表面に安定に露出することが明らかになった.

The effect of mass anisotropy on the thermoelectric figure of merit in the presence of polar optical phonon (POP) scattering is numerically studied. Electron transport is considered using the linear Boltzmann transport equation. The mass anisotropy is advantageous for the figure of merit along the principal directions of a small effective mass if phonon thermal conductivity is large. Inelasticity plays an essential role in the obtained result. The result is qualitatively different from that for an ionised impurity model.

The effect of scattering anisotropy caused by mass anisotropy on the thermoelectric figure of merit is investigated. The simplest electron transport model is considered assuming a single-band, parabolic energy dispersion relation and ionized impurity scattering, which is assumed to be elastic. The wave-vector dependence of the relaxation time is fully considered within the relaxation-time approximation. Transport coefficients are calculated using the anisotropic relaxation time at finite temperatures. The figure of merit perpendicular to the axis with the largest effective mass is shown to decrease as the effective mass ratio increases. This result indicates that the weak confinement of electrons into layers of bulk materials cannot be the direct cause of the enhancement of the figure of merit along the layers.

The effect of scattering anisotropy caused by mass anisotropy on the thermoelectric figure of merit is investigated. The simplest electron transport model is considered assuming a single-band, parabolic energy dispersion relation and ionized impurity scattering, which is assumed to be elastic. The wave-vector dependence of the relaxation time is fully considered within the relaxation-time approximation. Transport coefficients are calculated using the anisotropic relaxation time at finite temperatures. The figure of merit perpendicular to the axis with the largest effective mass is shown to decrease as the effective mass ratio increases. This result indicates that the weak confinement of electrons into layers of bulk materials cannot be the direct cause of the enhancement of the figure of merit along the layers.

We have extended the single impurity cluster model to include the effects of charge transfer to the conduction band. By introducing a few well defined parameters to describe the interaction between the single impurity and the conduction band we find an improved agreement for the asymmetric line shapes of the metal 2p core-level photoemission spectra of the 3d-transition metal pyrites FeS_2_ and CoS_2_ and of NiAs-type NiS.

We report the following two remarks of the Rh impurity effect on the charge-transfer (CT) type nonmetallic state (below the nonmetal-metal transition temperature: T_t_) of 3d transition metal compound NiS. (1) Although the nearest neighbouring M-M (Ni or Rh) and S-S distances increase with increasing x, dT_t_/dx<0. The Rh substitution gives a broad peak with the Fermi edge. A positive sign of T-linear contribution of the thermoelectric power turns to negative one at the critical concentration (x_c_) in Ni_1-x_Rh_x_S. Below T_t_, a Curie constant is observed and increases with increasing x. It is suggested that an impurity band introduced by the Rh atoms leads a change of the electronic state of Ni atoms and the suppression of the CT gap. (2) An empirical relation (dT_t_/dC_N_ >0) is found on Ni_1-x_Rh_x_S, Ni_1-δ_S and NiS_1-y_Se_y_, where C_N_ is the phonon-drag coefficient. The slope dT_t_/dC_N_ becomes smaller in Ni_1-x_M_x_S with the early 3d transition metal impurities (M: Ti, V and Cr).

We have extended the single-impurity cluster model to include the effects of charge transfer to the conduction band for the interpretation of the core level photoemission spectra of highly correlated, small-gap semiconducting and metallic transition metal compounds. The model includes the charge fluctuations between the local transition metal site and the conduction band and has been successfully applied to reinterpret the spectra of some 3d transition metal chalcogenides, reproducing well the large asymmetry of the main peaks, as well as the other satellite structures.

We have measured high-resolution photoemission spectra of NiS, which undergoes a nonmetal-metal transition at Tt~260 K. Below Tt, a small band gap opens; the band edge is essentially a step function broadened only by ~ 15 meV. This observation is difficult to reconcile with one-electron band theory, which predicts a much broader band edge, implying a dramatic influence of electron correlation on quasiparticle excitations and on the opening of a band gap in 3D antiferromagnetic insulators.

The M 2p core-level XPS peaks of doped elements in Ni1-xMxS are observed at the binding energies of M in “M2S3” rather than those of “MS” below and above the nonmetal-metal transition temperature (T t), indicating that the M atom are ionized to M3+ and acts as shallow donor impuritites. Below T t the T1/2 contribution with negative sign to the thermoelectric power (S) is superimposed on the usual degenerate semiconductive contribution with positive sign. It overcomes the latter contribution above a concentration (x c). The value of x c increases in the order of M=Ti, V and Cr. These results indicate that the T1/2 contribution to S comes from variable range hopping through the localized states formed near E F within the narrow charge-transfer gap of the parent NiS. Above T t the sign of S is negative in the electron-doped Ni1-xMxS as well as in the hole-doped Ni1-xS, suggesting that they have the Fermi surface topology.

X-ray photoemission spectroscopy (XPS) measurements of the valence band, Ni 2p core level, Co 2p core level and S 2s, 2p core levels were carried out on the metallic state of Ni1-vCovS (0≤ v≤ 0.06). The main peak of the valence band spectra shifts toward lower binding energy (E B) with increasing v ( dE B/ dv=-2.6± 0.4 eV/mol). This behavior is different from those reported on vacancy introduction or on the Se substitution in Ni1-xS or NiS1-y Sey ( dE B/ dxapprox 0; dE B/ dyapprox 0). The Ni 2p3/2 core level peak also shifts toward lower E B ( dE B/ dv=-1.8± 0.6 eV/mol), while no change in the S 2s or 2p core level peak is observed. We propose the possibility that the hybridization between the Co d7L and the Ni d8L peaks near the E F, where L denotes a ligand hole, leads to the modification of the electronic state of Ni sites in Ni1-vCovS.

We have observed small changes in the ultraviolet photoemission spectra of NiS across the temperature-induced phase transition between the Pauli-paramagnetic metal and the antiferromagnetic nonmetal. In going from the nonmetallic to the metallic phases, the main band (d8L) within ~3 eV below the Fermi level EF is shifted toward EF by 0.05 eV. This observation is consistent with the closing of a S 3p–Ni 3d charge-transfer gap and cannot be reconciled with that of a band gap of antiferromagnetic origin. The spectrum loses a small fraction of intensity in the vicinity of EF as the gap is collapsed. The virtually identical line shape of the main band between the two phases is consistent with the persistence of short-range antiferromagnetic correlation in the paramagnetic phase.