Shoji Ishibashi

A Possible Ground State and Its Electronic Structure of a Mother Material (LaOFeAs) of New Superconductors

The electronic and magnetic properties of the mother material LaOFeAs of new superconductors have been carefully studied using first-principles electronic structure calculations based on the generalized gradient approximation in the density functional theory. The present calculation predicts that the ground state of LaOFeAs is antiferromagnetic with a stripe type magnetic moment alignment leading to orthorhombic symmetry of the crystal. In this particular magnetic state, the density of states at the Fermi level is very small. On the other hand, LaOFeP has turned out to be paramagnetic and a good metal. Implications of the results regarding the experimental observations will also be presented.

Static magnetic order in La1.875Ba0.125CuO4

Zero field muon spin relaxation measurements of high quality, single phase La1.875Ba0.125CuO4 indicate that this system exhibits static magnetic order below 38 K. At this x = 18 concentration, superconductivity is suppressed. The ordered spins most likely reside on the copper sites.

A Single-Component Molecular Superconductor

The pressure dependence of the resistivities of a single-component molecular conductor, [Ni(hfdt)2] (hfdt = bis(trifluoromethyl)tetrathiafulvalenedithiolate) with semiconducting properties at ambient pressure was examined. The four-probe resistivity measurements were performed up to ∼10 GPa using a diamond anvil cell. The low-temperature insulating phase was suppressed above 7.5 GPa and the resistivity dropped, indicating the superconducting transition occurred around 7.5-8.7 GPa with a maximum Tc (onset temperature) of 5.5 K. The high-pressure crystal and electronic band structures were derived by the first-principle calculations at 6-11 GPa. The crystal was found to retain the semiconducting band structure up to 6 GPa. But the electron and hole Fermi surfaces appear at 8 GPa. These results of the calculations agree well with the observation that the pressure-induced superconducting phase of [Ni(hfdt)2] appeared just above the critical pressure where the low-temperature insulating phase was suppressed.

First-Principles Electronic Structure of Solid Picene

To explore the electronic structure of the first aromatic superconductor, potassium-doped solid picene which has been recently discovered by Mitsuhashi et al. with the transition temperatures T c =7–20 K, we have obtained a first-principles electronic structure of solid picene as a first step toward the elucidation of the mechanism of the superconductivity. The undoped crystal is found to have four conduction bands, which are characterized in terms of the maximally localized Wannier orbitals. We have revealed how the band structure reflects the stacked arrangement of molecular orbitals for both undoped and doped (K 3 picene) cases, where the bands are not rigid. The Fermi surface for K 3 picene is a curious composite of a warped two-dimensional surface and a three-dimensional one.

Weyl Node and Spin Texture in Trigonal Tellurium and Selenium.

We study Weyl nodes in materials with broken inversion symmetry. We find based on first-principles calculations that trigonal Te and Se have multiple Weyl nodes near the Fermi level. The conduction bands have a spin splitting similar to the Rashba splitting around the H points, but unlike the Rashba splitting the spin directions are radial, forming a hedgehog spin texture around the H points, with a nonzero Pontryagin index for each spin-split conduction band. The Weyl semimetal phase, which has never been observed in real materials without inversion symmetry, is realized under pressure. The evolution of the spin texture by varying the pressure can be explained by the evolution of the Weyl nodes in k space.

Vacancy-oxygen complexes and their optical properties in AlN epitaxial films studied by positron annihilation

Vacancy-type defects in AlN grown by metal-organic vapor phase epitaxy (MOVPE) and lateral epitaxial overgrowth (LEO) using halide vapor phase epitaxy were probed by a monoenergetic positron beam. Doppler broadening spectra of the annihilation radiation were measured and compared to the spectra calculated using the projector augmented-wave method. For MOVPE-AlN, the concentration of vacancy-type defects was high near the interface between AlN and the GaN buffer layer, and the defect-rich region expanded from the interface toward the surface when the NH3 flow rate increased. For the sample grown on the AlN buffer layer, however, the introduction of such defects was suppressed. For LEO-AlN, distinct deep emission peaks at 3–6 eV were observed in cathodoluminescence spectra. From a comparison between Doppler broadening spectra measured for LEO-AlN and computer simulated ones, an origin of the peaks was identified as complexes of Al vacancy (VAl) and oxygen atoms substituting nitrogen sites such as VAl(ON)n (...

Static dielectric response and born effective charge of BN nanotubes from ab initio finite electric field calculations

Ab initio investigations of the full static dielectric response and Born effective charge of BN nanotubes (BN-NTs) have been performed using finite electric-field method. It is found that the ionic contribution to the static dielectric response of BN-NTs is substantial and also that a pronounced chirality-dependent oscillation is superimposed on the otherwise linear relation between the longitudinal electric polarizability and the tube diameter

First-principles structural optimization and electronic structure of the superconductor picene for various potassium doping levels

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First-Principles Study of Magnetocrystalline Anisotropy and Magnetization in NdFe12, NdFe11Ti, and NdFe11TiN

, as for a thin dielectric cylindrical shell. In contrast, the transverse dielectric response of the BN-NTs resembles the behavior of a thin (nonideal) conducting cylindrical shell of a diameter of

Annealing properties of vacancy-type defects in ion-implanted GaN studied by monoenergetic positron beams

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