Published papers

We demonstrate the fabrication of superconducting SmFeAsO_1-x_F_x_ (Sm-1111) wires by using the ex-situ powder-in-tube technique. Sm-1111 powder and a binder composed of SmF3, samarium arsenide, and iron arsenide were used to synthesize the superconducting core. Although the F content of Sm-1111 is reduced in the process of ex-situ fabrication, the binder compensates by sufficiently supplementing the F content, thereby preventing a decrease in the superconducting transition temperature and a shrinkage of the superconducting volume fraction. Thus, in the superconducting Sm-1111 wire with the binder, the transport critical current density reaches the highest value of 4 kA/cm^2^ at 4.2 K.

The photoemission spectra of NiS in both antiferromagnetic and Pauli paramagnetic phases show similar characteristic features of localized d electrons. This finding is in contrast with the results of band-structure calculations which predict drastic spectral changes between the two phases. It is suggested that fluctuations of the Ni 3d local moments due to hybridization with S 3p states near the Fermi level are responsible for the Pauli paramagnetic behavior.

X-ray diffraction, electrical resistivity, DSC and XPS investigations were carried out on Cu4Sn1-xGexS4 (0≤x≤1.0). At room temperature, the orthorhombic structure changes to a monoclinic one at a transition concentration of xt=0.6 as x increases. This transition also exhibits the expansions in the energy width between the top of the valence band and the Fermi level and in the lattice parameter B. The transition has the characteristics of the first-order high (high temperature)-to-low (low temperature) conductivity phase transition as reported on Cu4SnS4. The transition temperature Tt linearly increases with x. These features are briefly compared with the electronic and structural phase transition reported on SnTe and Pb1-xGexTe.

In order to reveal the microscopic origin of the metal-nonmetal transition in NiS, we have studied the Ni 2p x-ray-absorption spectra of substituted systems Ni_1-x_M_x_S (M=Fe,Co), whose transition temperature T_t_ varies from 500 K (Ni_0.5_Fe_0.5_S) to below 4 K (Ni_0.94_Co_0.06_S). Spectra taken at room temperature have been analyzed using a configuration-interaction cluster model to deduce changes in the electronic structure parameters, namely, the S-3p-to-Ni-3d charge-transfer energy Δ and the Ni3d –S3p transfer integral T. We find that Fe substitution does not increase the local Ni-S distance as would be expected from the lattice parameters but affects energy levels as is reflected in Δ. The changes in Δ and T derived from the cluster-model analysis explain the increase in the charge-transfer-type band gap in Ni_1-x_Fe_x_S and hence the increase in T_t_ within the cluster model, whereas they do not explain the decrease in T_t_ in Ni_1-x_Co_x_S. In order to explain the dramatic decrease in T_t_ with Co substitution, we propose that the Ni3d–Co3d hybridization increases the bandwidths leading to the closure of the band gap.

The electronic structure of NiS on both sides of the metal-nonmetal transition has been studied by photoemission spectroscopy using synchrotron radiation and compared with band structure calculations. Interplay between Ni d-ligand hybridization, electron correlation, exchange interaction, and d-band formation is discussed.

The photoemission spectra of NiS, wchich exhits an antiferromagnetic nonmetal-to-a Pauli paramagnetic metal at T_t_=263K, have been measured. The spectra show similar characteristic features of localized d electrons, in contrast with the results of band-structure calculations which predict drastic spectral changes between the two phases.

Hexagonal lattice parameters a and c of the NiAs-type NiS_1-y_Se_y_ expand with increasing y below the solubility limit y_L_=0.20. The first-order transition temperature T_t_(y) [between the antiferromagnetic less-conductive and the Pauli-paramagnetichigh-conductive states] decreases with increasing y, and is suppressed down to 4.2K at the critical concentration y_c_=0.l33. Just above T_t_, the temperature slope of the susceptibility has positive sign at low y, and shows a y-dependence. The isothermal susceptibility of the Pauli-paramagnetic state decreases with increasing y. These experimental results are discussed on the basis of the strongly correlated features of d and p bands in NiS.

We have studied the dielectric responses of the layered cobalt oxysulfide Sr_2_Cu_2_CoO_2_S_2_ with the CoO_2_ square planes. With decreasing temperature below the Néel temperature, the resistivity increases like a semiconductor, and the thermopower decreases like a metal. The dielectric constant is highly dependent on temperature, and the dielectric relaxation is systematically changed with temperature, which is strongly correlated to the magnetic states. These behaviors suggest that carriers distributed homogeneously in the paramagnetic state at high temperatures are expelled from the antiferromagnetically ordered spin domain below the Néel temperature.

Polycrystalline Ca and F co-doped LaFeAsO samples (La1-xCaxFeAsO1-yFy) were synthesized by solid-state reactions. Samples with nominal composition: (y - x) ≤ 0.05 did not show superconductivity at temperatures T > 4.2 K, although an anomalous kink of the ρ-T curves, which is related to crystallographic phase transitions, was observed at T ∼ 150 K. Samples with nominal composition: (y - x) ≥ 0.15 did exhibit superconductivity. The superconducting transition temperature (T c) of co-doped LaFeAsO with nominal composition: (x, y) = (0.25, 0.50) was determined to be T c = 31.5 K, which is higher than the highest T c value observed for LaFeAsO1-xFx materials synthesized under ambient pressure.

The advanced data processing on the edge devices is getting much attention in addition to the performance of cloud computing for big data analysis and artificial intelligence for the advanced cyber-physical systems and Internet of Things. This paper shows the challenges of research and development on disruptive computing which enhance the conventional computing system with high performance and low power consumption. Future R&D challenges in computing technologies involve all technology layers including algorithms, packaging, software, architecture, circuits, devices, and materials. It is important from the perspective of social implementation and efficiency to select technologies suitable for important applications from each technology layer specific to the computing domain and with consideration for the timeline, and to develop technologies through a vertical integration approach. The promoting method of implementing such R&D is also described.

Electrical and thermal transport properties of a polycrystalline carrier-doped wide-gap semiconductor LaCu1-δ S0.5Se0.5O (δ = 0.01), in which the CuCh (Ch = S, Se) layer works as conducting layer, were measured at temperatures 473~673 K. The presence of δ = 0.01 copper defects dramatically reduces the electrical resistivity (ρ) to approximately one part per million compared to that of δ = 0 at room temperature. The polycrystalline δ = 0.01 sample exhibited ρ of 1.3 × 10-3 Ωm, thermal conductivity of 6.0 Wm-1 K-1, and Seebeck coefficient (S) of 87 µVK-1 at 673 K. The maximum value of the dimensionless figure of merit (ZT) of the δ = 0.01 sample was calculated to be 6.4 × 10-4 at T = 673 K. The ZT value is far smaller than a ZT ~ 0.01 measured for a nominal LaCuSeO sample. The smaller ZT is mainly due to the small S measured for LaCu1-δS0.5Se0.5O (δ = 0.01). According to the Debye model, above 300 K phonon thermal conductivity in a pure lattice is inversely proportional to T, while thermal conductivity of the δ = 0.01 sample increases with increasing T.

有機金属堆積法を用いてY0.77Gd0.23Ba2Cu3O7-delta ((Y,Gd)BCO) をSrTiO3基板上に成長させた薄膜試料を用意した. これらの試料を水素雰囲気下, および 水素負イオン照射下で熱処理を行い, その結晶相の安定性を評価した. いずれの熱処理条件でも300℃の熱処理では(Y,Gd)BCOの結晶相は消失した. アンダードープY,Gd)BCOでは250℃の熱処理でも(Y,Gd)BCOの結晶相が維持された. 200℃のいずれの熱処理条件でも、試料回収後の室温での格子定数cが増加した. 210℃での水素負イオン照射下熱処理を施した試料のc軸長は減少した. 210℃での水素雰囲気下熱処理試料の格子定数は, 同様にcが像がした. これに対し210℃での水素負イオン照射下熱処理を施した試料のc軸長は減少した.

The thermoelectric transport properties of the van der Waals-type layered rhombohedral SnAs-based compound, EuSn2As2, have been investigated. A densified polycrystalline sample of EuSn2As2 with porosity (φ) of 2.4(9) vol.% exhibited a weak orientation to the c-axis for hexagonal coordination system; the weak orientation is parallel (P ∥) to the pressing direction of hot pressing. Measurements of electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) were conducted perpendicular (P ⊥) to the pressing direction. The experimental values of ρ and S exhibit metallic temperature dependence and p-type carrier polarity. The power factor (P) was 0.51(8) mW m-1 K-2 at 673(4) K. Using the Wiedemann-Franz-Lorenz law, the phonon thermal conductivity (κ ph) was estimated to be 0.4(6) W m-1 K-1 at 673(6) K. The dimensionless figure of merit, ZT, was 0.092(17) at 673(3) K.