Showing papers by "Satoru Matsuishi published in 2015"

Superconductivity in room-temperature stable electride and high-pressure phases of alkali metals.

Abstract: S-band metals such as alkali and alkaline earth metals do not undergo a superconducting transition (SCT) at ambient pressure, but their high-pressure phases do. By contrast, room-temperature stable electride [Ca24Al28O64]4+⋅4e− (C12A7:e−) in which anionic electrons in the crystallographic sub-nanometer-size cages have high s-character exhibits SCT at 0.2–0.4 K at ambient pressure. In this paper, we report that crystal and electronic structures of C12A7:e− are close to those of the high-pressure superconducting phase of alkali and alkaline earth metals and the SCT of both materials is induced when electron nature at Fermi energy ( E F) switches from s- to sd-hybridized state.

Superconductivity at 52 K in hydrogen-substituted LaFeAsO1-xHx under high pressure

Abstract: The 1111-type iron-based superconductor LnFeAsO1-xFx (Ln stands for lanthanide) is the first material with a Tc above 50 K, other than cuprate superconductors. Electron doping into LaFeAsO by H, rather than F, revealed a double-dome-shaped Tc-x diagram, with a first dome (SC1, 0.05 50 K observed in RE-1111 compounds (RE: Pr, Sm, and Gd) at ambient pressure is the merging of SC1 and SC2.

Superconductivity in Room Temperature Stable electride and high-pressure phases of alkali metals

Abstract: S-band metals such as alkali and alkaline earth metals do not undergo a superconducting transition (SCT) at an ambient pressure, but their high-pressure phases do. In contrast, room temperature stable electride electride (C12A7:e-) in which anionic electrons in the crystallographic sub-nanometer-size cages have high s-character exhibits SCT at 0.2-0.4K at an ambient pressure. In this paper we report that crystal and electronic structure of C12A7:e- are close to those of the high pressure superconducting phase of alkali and alkaline earth metals and the SCT of both materials is induced when electron nature at Fermi energy (EF) switches from s- to sd-hybridized state.

Narrow Bandgap in beta-BaZn2As2 and Its Chemical Origins

Abstract: Beta-BaZn2As2 is known to be a p-type semiconductor with the layered crystal structure similar to that of LaZnAsO, leading to the expectation that beta-BaZn2As2 and LaZnAsO have similar bandgaps; however, the bandgap of beta-BaZn2As2 (previously-reported value ~0.2 eV) is one order of magnitude smaller than that of LaZnAsO (1.5 eV). In this paper, the reliable bandgap value of beta-BaZn2As2 is determined to be 0.23 eV from the intrinsic region of the tem-perature dependence of electrical conductivity. The origins of this narrow bandgap are discussed based on the chemi-cal bonding nature probed by 6 keV hard X-ray photoemission spectroscopy, hybrid density functional calculations, and the ligand theory. One origin is the direct As-As hybridization between adjacent [ZnAs] layers, which leads to a secondary splitting of As 4p levels and raises the valence band maximum. The other is that the non-bonding Ba 5dx2-y2 orbitals form unexpectedly deep conduction band minimum (CBM) in beta-BaZn2As2 although the CBM of LaZnAsO is formed mainly of Zn 4s. These two origins provide a quantitative explanation for the bandgap difference between beta-BaZn2As2 and LaZnAsO.

Hydrogen-Substituted Superconductors SmFeAsO1–xHx Misidentified As Oxygen-Deficient SmFeAsO1–x

Abstract: We investigated the preferred electron dopants at the oxygen sites of 1111-type SmFeAsO by changing the atmospheres around the precursor with the composition of Sm:Fe:As:O = 1:1:1:1 – x in high-pressure synthesis. Under H2O and H2 atmospheres, hydrogens derived from H2O or H2 molecules were introduced into the oxygen sites as a hydride ion, and SmFeAsO1–xHx was obtained. However, when the H2O and H2 sources were removed from the synthetic process, nearly stoichiometric SmFeAsO was obtained and the maximum amount of oxygen vacancies introduced remained x = 0.05(4). Density functional theory calculations indicated that substitution of hydrogen in the form of H– is more stable than the formation of an oxygen vacancy at the oxygen site of SmFeAsO. These results strongly imply that oxygen-deficient SmFeAsO1–x reported previously is SmFeAsO1–xHx with hydride ion incorporated unintentionally during high-pressure synthesis.

Low dimensionalization of magnetic ordering in S r 2 V O 4 by hydride ion substitution

Abstract: Substitution of an oxygen anion with a hydrogen anion induced the low dimensionalization of magnetic ordering in a transition metal oxide $\mathrm{S}{\mathrm{r}}_{2}\mathrm{V}{\mathrm{O}}_{4}$. Upon increasing $x$ up to $\ensuremath{\sim}1$ in $\mathrm{S}{\mathrm{r}}_{2}\mathrm{V}{\mathrm{O}}_{4\ensuremath{-}x}{\mathrm{H}}_{x}$, the hydride ions were ordered linearly, and the magnetic susceptibility was simultaneously suppressed. It was found that this suppression was attributed to the formation of a quasi-one-dimensional antiferromagnetic spin chain, maintaining that each of the vanadium cations is two-dimensionally bridged by hydride and oxide ions. Density functional theory calculations demonstrate that the quasi-one-dimensional property is caused by much enhanced anisotropic exchange couplings $({J}_{1}/{J}_{2}\ensuremath{\sim}6)$ originating from the absence of $\ensuremath{\pi}$ bonding between $\mathrm{H}\phantom{\rule{0.28em}{0ex}}1s$ and $\mathrm{V}\phantom{\rule{0.28em}{0ex}}3d$ orbitals. Utilizing a hydride ion that has an ionic radius similar to an oxygen anion and only one energetically available orbital of $1s$ is a different approach to realization of magnetic low dimensionalization in $3d$ early transition metal oxides.

Two-Dimensional Transition-Metal Electride Y2C.

Abstract: Polycrystalline layered Y2C with quasi-2D anionic electrons confined in the interlayer is prepared by melting stoichiometric amounts of Y shots and C granules under Ar.

Crystal structure and physical properties of new transition metal based pnictide compounds: LaTM2AsN (TM = Fe, Co, and Ni)

Abstract: New 3d transition metal-based mixed-pnictide compounds, LaTM2AsN (TM = Fe, Co, and Ni) are synthesized by solid state reactions under a high pressure of 2.5 GPa. These compounds crystallize with an orthorhombic structure (space group Cmcm) containing four formula units per unit cell. The crystal structure consists of an anisotropic network of TMAs3N tetrahedra sharing As-As edges along the in-plane ac direction and N corners along the b-direction, forming a TM honeycomb lattice with a boat-shape conformation bridged by TM-N-TM linear bonds. The temperature dependences of the electrical resistivity and magnetic susceptibility indicate that these crystals are itinerant antiferromagnets exhibiting parasitic ferromagnetism with transition temperatures of 560, 260, and 410 K for TM = Fe, Co, and Ni, respectively. These compounds are expected to be parent materials for new superconductors.

Superconductivity at 52 K in hydrogen-substituted LaFeAsO1-xHx under high pressure

Abstract: The 1111-type iron-based superconductor LnFeAsO1-xFx (Ln stands for lanthanide) is the first material with a Tc above 50 K, other than cuprate superconductors. Electron doping into LaFeAsO by H, rather than F, revealed a double-dome-shaped Tc-x diagram, with a first dome (SC1, 0.05 50 K observed in RE-1111 compounds (RE: Pr, Sm, and Gd) at ambient pressure is the merging of SC1 and SC2.