Published papers

Undoped LaCuSeO is an electrically insulating mixed anion layered compound (MALC). Anisotropic optical band structure of uniaxially hot-pressed (HP) polycrystalline LaCuSeO was demonstrated. It was defined that directions for the measurements parallel/vertical to the pressing axis as P// / P⊥. And we also defined a pellet which was not hot pressed and as-grown, as Undoped. The electronic properties of LaCuSeO were examined by measurement of reflectivity and analysis using Kubelka-Munk equation and Tauc plot. Optical band gap of Undoped is 2.7 eV, and those of P// and P⊥ are 2.6 eV. Activation energies were 0.63 eV for Undoped, 0.26 eV for P//, and 0.36 eV for P⊥.

In 2012, Kitano et al. reported that a Ru-loaded electride, [Ca24Al28O64]4+(e-)4 (Ru/C12A7:e-), works as an efficient catalyst for synthesis of NH3. The C12A7:e-, the first room temperature stable electride, exhibits a low work function (2.4 eV) comparable to that of potassium metal, with chemically and thermally stability. In this work, work function of C12A7:e- was calculated based on density functional theory (DFT). Moreover, atomic configurations of Ru/C12A7:e- surfaces were predicted using slab model so as to investigate the mechanism of the catalytic reaction of ammonia synthesis.

Distribution of radioactive 134Cs due to the Fukushima Dai-ichi Nuclear Power Plant accident was measured using a scintillation spectrometer in the mountainous areas of southern part of the Tohoku, northern and southern parts of Kanto and Koshin-etsu regions. As a result, intensity of radiation increased with decrease of altitude in Mt. Azuma and Mt. Yamizo, whereas intensity increased with increase of altitude in Mt. Gozen and Mt. Oyama. This indicates that plumes (polluted air masses) with their center at the specific height moved depending on the meteorological condition. Although radioactive 134Cs was not detected at Mt.Kaikoma, relatively intense γ-ray was observed due to radioactive 40K from granite

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.

Superconducting wires were fabricated via an ex-situ powder-in-tube (PIT) process using a Fe/Ag bimetallic sheath as a sheath material and Sr2VFeAsO3-δ as a core material. The superconducting wires were prepared by a solid-state reaction in an evacuated quartz tube at 900C for 1, 2, and 4 h after being rolled. The superconducting wires exhibited zero resistivity at temperatures  8-12 K. Critical current densities (Jc) of the superconducting wires were determined from their V-I curves. The Jc was extrapolated up to 45.3 Acm-2 at 0 K for the 1 h-sintered wire.

SnAs-based layered EuSn2As2 is a candidate for a thermoelectric material due to its “so-called Zintl-phase-like” crystallographic structure. Its crystallographic structure consists of isolated Eu2+ cations and [Sn2As2]2- anions bilayers, which is bound by van der Waals forces. Polycrystalline EuSn2As2 was prepared by two synthesis procedures. In a procedure, a polycrystalline sample was prepared from a europium (Eu) ingot and tin (Sn) and arsenic (As) powders in an alumina tube sealed in an evacuated silica tube. In the other procedure, a sample was prepared from a Eu ingot and a Sn-As pellet in a carbon crucible in an evacuated silica tube. EuSn2As2 was obtained as a dominant phase by both of the procedures, although the purest polycrystalline sample was obtained by the latter procedure.

Undoped LaCuSeO is an electrically insulating mixed anion layered compound (MALC) with an optical band gap Eg ~2.7 eV. Anisotropic thermoelectric exchange properties of uniaxially hot-pressed (HP) polycrystalline LaCuSeO, which is a representative MALC with ZrCuSiAs-type structures, are demonstrated. We defined directions for the measurements parallel/vertical to the pressing axis as P// / P⊥. A crystallographic anisotropy of the polycrystalline LaCuSeO is verified by a ratio between Bragg diffraction intensities with Miller indices 003 and 110 in a tetragonal lattice. XRD patterns of P// and P⊥ show weakly a-axis and c-axis oriented crystallographic phases. While undoped LaCuSeO shows a high resistivity with rho = 1.0・102 Ohm m and does not show measureable Seebeck coefficient at room temperature, rho of the P// and P⊥ show ~60 times smaller than that of undoped. P// and P⊥ exhibit a finite large Seebeck coefficient (S) with ~800 micro VK-1 at T ~400 K . The smaller rho are mainly due to a hole doping by slight crystallographic defects induced via hot-pressing process.

Layered mixed-anion compounds with ZrCuSiAs-type structure exhibit various electronic functional properties such as p-type semi-conducting properties, high thermoelectric performance, and high superconducting transition temperature (Tc) superconductivity. After the discovery of an iron-based superconductor, LaFeAsO1-xFx with Tc at 26 K, the mixed anion compounds have been expected as mother compounds for novel superconducting materials. In LaFeAsO1-xFx, the appearance of superconducting phases is simultaneously occurred with quenching of spin polarization in Fe ions. In this study, density functional theory (DFT) calculation of LaFeAsO1-xFx (x = 0.000-0.125) was performed by using Vienna Ab−initio Simulation Package (VASP) code. Our calculation roughly reproduces Ishibashi’s report, which demonstrates the most stable antiferromagnetic phase with stripe magnetic ordering in orthorhombic LaFeAsO, and experimental magnetic phase in LaFeAsO1-xFx. Theoretical spin polarization of Fe almost disappears at x  0.042 in LaFeAsO1-xFx. These results are consistent with electronic magnetic phase diagrams for LaFeAsO1-xFx. Keywords: Iron-Based superconductor, Density functional theory, Antiferromagnetic ordering, Magnetic moment, spin polarization, LaFeAsO, LaFeAsO1-xFx

We report the results of 31P-nuclear magnetic resonance (NMR) measurements on the layered cobalt oxyphosphide Sr2Sc(Co1-xFex)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-xFex)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=T1 is almost proportional to the temperature at low temperatures. The magnitude of 1=T1T decreases with increasing the Fe content, which suggests the decrease of the density of states around the Fermi level.

Polycrystalline samples of novel oxypnictides, Sr2ScCo1-xFexPO3, with Fe-doped Co1-xFexP layer were synthesized, and their resistivities and Seebeck coefficients were measured. These materials, Sr2ScCo1-xFexPO3, crystallize in a layered structure which represent an alternative stack of ThCr2Si2 type Co1-xFex layers and K2NiF4 type Sr2ScO3 layers. The resistivities 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 lower temperature.

新規な超伝導体であるLaPt5Asを発見した。この構造はカワイ型高圧セルを用いた高圧合成によってのみ得られる。LaPt5Asの超伝導転移温度は2.6 Kである。この物質の結晶相は熱処理温度に依存し、10 GPa付近での合成により単相な試料が得られる。LaPt5Asのc軸長は6 nmを超え、非常に長い。密度汎関数理論による電子状態計算によると、LaPt5Asの電気伝導はPtによる金属結合ネットワークを介して実現していることが予想された。

Mn3O4は15 GPa 以上の高圧下で、スピネル構造からポストスピネル構造に転移し、低温から室温にかけて、逐次結晶構造相転移が生じることがHiraiらにより確かめられている。本論文は、粉末の反射率分光法、及び密度汎関数理論を用いてポストスピネル型Mn3O4の電子磁気構造を、実験的、理論的に明らにする。Mn3O4は絶縁体であり、伝導体に存在するエネルギーの異なる２つの状態密度の存在は、実験的にも示された。

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。

量子相転移(QCP)は、磁性に近接し、しばしば超伝導相の出現を伴うことから、多くの研究者に着目される物理現象である。本報告では、重い電子系で生じる強磁性的なQCPに関して予備的な報告を行う。Ce(Fe1-xCrx)PO (x = 0.10, 0.20, 0.30)多結晶試料は、固相反応により作製された。各試料の結晶相はX線回折により確認した。各試料の磁気転移温度はアロットプロットにより求めた。Ce(Fe1-xCrx)POのQCPは、x=0.00-0.10に存在する。

地殻に多く含まれている元素である銅を主成分として含む硫化物は、電子機能が発現する場合、コスト上魅力的な材料といえる。本稿では、室温から573 KでのCu3SbS4の熱電パラメータ(電気抵抗率、熱伝導率、ゼーベック係数)を測定し、その無次元性能指数(ZT)は、Snドープにより0.1程度まで上昇することを報告する。

シャンド鉱型強磁性体Co3-xFexSn2S2の室温から1673 K、常圧から11 GPaの高温高圧において、その場エネルギー分散X線回折パターンを測定し、Feの固溶限界の圧力依存性を明らかにした。常圧下ではFeの固溶はx=0.6が上限であるのに対し、高圧下ではx=1.0程度まで固溶限界が上昇する。それに伴い、強磁性転移温度は、母相の175 Kよりx=1.0の68 Kまで減少する。これらの結果は擬ポテンシャルによる密度汎関数法による電子状態計算結果と矛盾しない。