Published papers

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 bulk superconductivity at temperatures T> 4.2 K, although an anomalous kink of the rho–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 bulk superconductivity. The superconducting transition temperature (Tc) of co-doped LaFeAsO with nominal composition: (x, y)= (0.25, 0.50) was determined to be Tc= 31.5 K, which is higher than the highest Tc value observed for LaFeAsO1-xFx materials synthesized under ambient pressure.

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.

© 2019 Author(s). Spectroscopic techniques, including photoelectron spectroscopy, diffuse reflectance, and x-ray absorption, are used to investigate the electronic structure and the local structure of LaOBiS 2 - xSe x thermoelectric material. It is found that Se substitution effectively suppresses local distortion that can be responsible for the increased carrier mobility together with a change in the electronic structure. The results suggest a possible way to control thermoelectric properties by tuning of the local crystal structure of these materials.

We have been investigating the effect of (CoFe2O4)x and (ZnFe2O4)x (x is the wt%) nano-sized ferrites on the superconducting (SC) properties of the high-Tc SC compound GdBa2Cu3O7-δ (Gd123) prepared by the conventional solid-state reaction technique. In this work, the magnetic hysteresis for pure Gd123 samples and samples added with nano-sized ferrites was measured using a superconducting quantum interference device (SQUID) magnetometer using a dc field up to 30,000 Oe. The magnetic Jc was estimated using the extended Bean model. While a clear increase in Jc values with the addition of (CoFe2O4)x nano-sized ferrites is observed, a less clear positive effect on the Jc values is observed with the addition of (ZnFe2O4)x nano-sized ferrites. These results indicate an improvement of grain connectivity, decrease of voids between the grains, and decrease of porosity in Gd123 compound resulting from CoFe2O4 addition consistent with the mechanical properties measured before. (CoFe2O4)x nano-sized ferrite addition has thus resulted in an improvement in the SC properties of Gd123 high-Tc SC compound making it more convenient for applications.

We elucidate the magnetic phases and superconducting transition temperatures (Tc) in Sr2VFeAsO3-delta (21113V), an iron-based superconductor with a thick-blocking layer fabricated from a perovskite-related transition metal oxide. At low temperatures (T < 37.1 K), 21113V exhibited a superconducting phase in the range 0.031 =< delta =< 0.145 and an antiferromagnetic (AFM) iron sublattice in the range 0.267 =< delta =< 0.664. Mixed-valent vanadium exhibited a dominant AFM phase in 0.031 =< delta =< 0.088, and a partial ferrimagnetic (Ferri.) phase in the range 0.124 =< delta =< 0.664. The Ferri. phase was the most dominant at a delta value of 0.267, showing an AFM phase of Fe at T < 20 K. Increasing the spontaneous magnetic moments reduced the magnetic shielding volume fraction due to the superconducting phase. This result was attributed to the magnetic phase of vanadium, which dominates the superconductivity of Fe in 21113V. The Tc-delta curve showed two maxima. The smaller and larger of Tc maxima occurred at delta = 0.073 and delta = 0.145, respectively; the latter resides on the phase boundary between AFM and the partial Ferri. phases of vanadium. 21113V is a useful platform for verifing new mechanisms of Tc enhancement in iron-based superconductors.

混合アニオン層状化合物の一種であるSr2VFeAsO3-deltaは酸素量の変化により, その超伝導転移温度が変化することが知られている。本論文では、Sr2VFeAsO3-deltaの酸素発生反応、すなわち水分解反応の触媒としての機能を新たに報告する。さらに計算機シミュレーションにより、水分解に寄与する結晶学的な特徴を明らかにした。

We have implemented laser photoemission spectroscopy (PES) to investigate the electronic structure of the iron-based superconductor (La,Eu)FeAsO1-xFx (LaEu1111) which is an interesting compound in the “1111” family showing a high value of the superconducting (SC) transition temperature (Tc) due to Eu doping. At least two energy scales were observed from the PES data in the SC compound: One at ~14 meV closing around Tc and thus corresponding to the SC gap. Another energy scale appears at ~35 meV and survives at temperatures above Tc which represents the pseudogap (PG). The non-SC sample (La,Eu)FeAsO shows a PG at ~41 meV. These observations in this new superconductor are consistent with the general trend followed by other compounds in the “1111” family.

© Published under licence by IOP Publishing Ltd. We report the results of 31P-nuclear magnetic resonance (NMR) measurements on the layered cobalt oxyphosphide Sr2Sc(Co1-xFe x )PO3 in order to investigate the magnetic properties at low temperatures from a microscopic view point. The 31P-Knight shifts measured at the resonance peak maximum of Sr2Sc(Co1-xFe x )PO3 have positive values and are T-independent in an entire temperature range, and the absolute value decreases with increasing Fe content. Also, the nuclear spin-lattice relaxation rate 1/T 1 is almost proportional to the temperature at low temperatures. The magnitude of 1/T 1 T decreases with increasing the Fe content, which suggests the decrease of the density of states around the Fermi level.

©2018 The Physical Society of Japan Inspired by the recent first-principles calculations showing the high thermoelectric performance of layered pnictogen chalcogenides, we experimentally characterise the crystal structure and high-temperature transport properties of the layered antimony selenides REOSbSe2 (RE = La, Ce). The crystal structure of REOSbSe2 was the tetragonal P4=nmm space group, consisting of alternate stacks of SbSe2 and REO layers. These two compounds were n-type semiconductors. The optical band gaps of LaOSbSe2 and CeOSbSe2 were evaluated to be 1.0 and 0.6 eV, respectively. The room-temperature thermal conductivity was 1.5 W m−1 K−1 for RE = La and 0.8 W m−1 K−1 for RE = Ce. These relatively low thermal conductivities were comparable to those of isostructural layered bismuth chalcogenides. We substituted O2− with F− ions to introduce electrons as charged carriers to optimize the thermoelectric performance, but increasing the electrical conductivity was still challenging.

© Published under licence by IOP Publishing Ltd. Polycrystalline samples of novel oxypnictides, Sr 2 ScCo 1-x Fe x PO 3 , with Fe-doped Co 1-x Fe x P layer were synthesized, and their resistivities and Seebeck coefficients were measured. These materials, Sr 2 ScCo 1-x Fe x PO 3 , crystallize in a layered structure which represent an alternative stack of ThCr 2 Si 2 type Co 1-x Fe x layers and K 2 NiF 4 type Sr 2 ScO 3 layers. The resistivities (ρ) decrease with decreasing temperature like a metal. The Seebeck coefficients (S) show negative values at room temperature, while the S show positive values for x = 0.04-0.7 at low temperature.

© 2019 The Japan Society of Applied Physics. Superconducting critical current density (J c) at 4.2 K for superconducting Sr2VFeAsO3-δ tape fabricated by powder-in-tube process was enhanced up to 285 A cm-2. Cross-sectional SEM images for superconducting Sr2VFeAsO3-δ wire/tapes indicated that void fraction f void was reduced from 12.8% for the round wire to ∼2.5% for the tapes. The decrease in f void suppresses the so-called "weak-links" and therefore enhances the J c. In order to enhance J c, the annealing condition should be optimized. The optimal annealing condition enforces the link between grains and avoids the loss of oxygen atoms in the superconducting core.

The effect of Arsenic substitution on the properties of Bi-2212 superconductors was studied. Solid-state reaction technique was used for the synthesis of Bi2−xAsxSr2Ca1.1Cu2.1O8 samples, x = 0, 0.1, 0.2 and 0.4. The X-ray diffraction confirmed the formation of the tetragonal phase of Bi-2212 superconductors. The decrement in the c parameter confirmed that As insertions were introduced into the Bi sites of the system. The grain morphology of the surface was analysed by scanning electron microscopy, where the amount of arsenic increases with As substitution. The superconducting transition temperature (Tc) was determined from the DC-electrical resistivity by the standard four-probe technique. Tc was enhanced up to x = 0.1 above which it was suppressed. For magnetism, half magnetic-hysteresis loops were obtained by SQUID magnetometer at different temperatures (5 and 30 K). The hysteresis loops sizes increased with increasing Asconcentrations. The critical current density (Jc) and the flux pinning mechanisms were investigated via different models.

Semiconducting mixed anion layered compounds (MALC) LaCu1-delta S0.5Se0.5O are synthesized via solid state reaction. We synthesized several nominal LaCu1-deltaS0.5Se0.5O (delta = 0, delta~ 0.01, and delta~ 0.02). A degenerated semiconducting bulk LaCu1-deltaS0.5Se0.5O appears at delta~ 0.01. p-type carriers are doped due to Cu deficient Cu1-deltaS0.5Se0.5 layers in LaCu1-deltaS0.5Se0.5O. In-gap states, which dominate electrical conduction, are observed at energy ~0.5 eV higher than that of the valence band in an energy band diagram for delta~ 0.01.

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 (phgr) 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 (κph) 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.

Superconducting critical current density (Jc) at 4.2 K for superconducting Sr2VFeAsO3-δ tape fabricated by powder-in-tube process was enhanced up to 285 Acm-2. Cross-sectional SEM images for superconducting Sr2VFeAsO3-δ wire/tapes indicated that void fraction f void was reduced from 12.8% for the round wire to ~2.5% for the tapes. The decrease in f void suppresses the so-called "weak-links" and therefore enhances J c. In order to enhance J c, the annealing condition should be optimized. The optimal annealing enforces the link between grains and avoids the loss of oxygen atoms in the superconducting core

Sr2VFeAsO3−delta (21113V) is a mixed anion layered compound containing perovskite-related vanadium (V) oxide blocking layer in unit cell. We theoretically demonstrate magnetic phase diagrams of V and Fe in 21113V (delta = 0, 0.25, 0.50)verified on density functional theory (DFT). heoretical magnetic phases of V are antiferromagnetic ordered phases (A-AF) for delta = 0 and ferrimagnetic ordered phases (Ferri.) for delta = 0.25, 0.50. Especially, 21113V for delta= 0.25 exhibits different spontaneous magnetizations (MS), and their formation energyisvery close.The stable magnetic phases of Fe in 21113V are stripe-type antiferromagnetic ordered phases (s-AF) in delta = 0, 0.25, 0.50, although the magnetic phase of the Fe in delta = 0.25 shows almost the same formation energy between s-AF and paramagnetic (PM) ordered phases. Theoretical magnetic ordered phases are qualitatively consistent with the experimental magnetic phases for 21113V, although quantitative differences appear between measuredelement specific magnetic moments and theoretical ones.

Sr2VFeAsO3−δ(21113V) is a mixed anion layered compound containing a perovskite-related vanadium (V) oxide blocking layer in a unit cell. In the previous study, the magnetic stability of V and Fe in 21113V (delta = 0, 0.25, 0.50) was theoretically verified on the density functional theory (DFT). In this study, we demonstrated the most stable site positions in the crystal structure including deficient oxygen. In delta = 0.25, 0.50, the z position of oxygen is reproduced reported results, preciously. It is indicated that oxygen deficiency is inevitable in the reported 21113V with nominally delta = 0 samples. The oxygen deficiency of the apical sites for the perovskite-related structure expands the lattice constant c. We also demonstrated the density of states (DOS) of 21113V. Fe in 21113V contributes to electrical conduction and V does not contribute to electrical conduction. Except for delta = 0, V in 21113V exhibits the ferrimagnetic phase (Ferri.) with spontaneous magnetic polarization.

Sr2VFeAsO3−delta (21113V) isa mixed anion layered compound containing a perovskite-related vanadium (V) oxide blocking layer in a unit cell.In 2019, electronic and magnetic phase diagrams as a function of oxygen deficiency (delta) on superconducting 21113V were experimentally demonstrated.In this study, the magnetic stability of V and Fe in 21113V (delta = 0, 0.25, 0.50) was theoretically verified on density functionaltheory (DFT). The deficient oxygen in apical sites of the perovskite-related structures is more stable than other deficient oxygen sites. The most stable magnetic phase of V in 21113V of delta = 0 is“antiferromagnetic ordered phase between two ferromagnetic V magnetic mono-layers”(A-AF).The A-AF of V transfers to ferrimagnetic ordered phases (Ferri.) in delta = 0.25, 0.50. Theoretical magnetic ordered phases are qualitatively consistent with experimental electronic and magnetic phases for 21113V, although the quantitative differences between theoretical element-specific magnetic spin moments and experimental ones should be discussed in later reports.

An electronic and magnetic phase diagram of a mixed anion layered compound, Sr2CrFeAsO3−δ, is demonstrated. Sr2CrFeAsO3−δ is composed by a carrier conducting FeAs layer and carrier blocking Sr2CrO3−δ layer. Polycrystalline Sr2CrFeAsO3−δ samples were prepared by solid-state reaction. The oxygen deficiency (δ) of the samples was determined using an assumption of a linear relation between δ and lattice volume (V). The δ-V relation was based on the V of several nominal compositions of polycrystalline samples with a smaller second phase. An electronic and magnetic phase diagram of Sr2CrFeAsO3−δ that considers δ and temperature (T) was created on the basis of electrical resistivity measurements and 57Fe Mössbauer spectroscopy measurements. A magnetic Fe sublattice of Sr2CrFeAsO3−δ exhibited a stripe-type antiferromagnetic (AFM-s) phase for 0.124 ≤ δ ≤ 0.210 at T < 60 K and an internal magnetic field (Hint) distributed antiferromagnetic (AFM-DH) phase for 0.247 ≤ δ ≤ 0.256 at T < 53 K.

銅欠陥のあるクラマイト型Cu3-xSnS4 (CTS)の電気/熱輸送特性を明らかにした。CTSの結晶学的な特徴はCuとSnのサイト間の構成イオンの部分的な交換(アンチサイト)である。CTSはバンド絶縁体的な挙動を示すCu2ZnS4とCu3SbS4と異なり, キャリアドープにより 10の21乗程度のキャリア密度を示す金属であり, 格子熱伝導率は 2 Wm-1K-1であった.この輸送現象はCu 3d と S 3p 軌道に由来する.