Masaaki Geshi

New half-metallic materials with an alkaline earth element

New candidates for half-metallic materials were theoretically designed recently by Geshi et al. The materials are calcium pnictides, i.e. CaP, CaAs and CaSb. When the zinc-blende structure was assumed, these compounds showed half-metallic electronic band-structure, in which a curious flat band was found. To explain this magnetism, we investigated characters of orbitals on this flat band of CaAs. The hybridization of p states of As with d states of Ca is shown to be essential for formation of a flat band made of localized orbitals. The appearance of complete spin polarization in the flat band suggests that the flat-band mechanism is relevant for the ferromagnetism. A connection from the first-principles result to a solvable Hubbard model with a flat band is discussed.

Half-Metallic p-Electron Ferromagnetism in Ca and Sr Pnictides

We investigated the magnetism in Ca and Sr pnictides by using the first-principles calculations. These compounds are half-metallic and ferromagnetic (FM) when they assume the zinc-blende structure at the equilibrium lattice constant. Ferromagnetism is induced by the spin polarization of the p -orbitals of the pnictogen atoms; Ca and Sr atoms have no magnetic moments, which is different from that of CrAs or CrSb with a zinc-blende structure. To confirm the mechanism of the ferromagnetism, we have calculated a hypothetical crystal – fcc-As with two additional electrons – and have shown that fcc-As has the same magnetic moment as CaAs with a zinc-blende structure. This means that the role of Ca or Sr atoms is to provide electrons with As atoms at the fcc site and to sustain the distances between the As atoms and crystal symmetry. The FM exchange interactions between the pnictogen atoms are considered to exist in these lattices, which is briefly discussed.

Million-Atom Molecular Dynamics Simulation by Order-N Electronic Structure Theory and Parallel Computation

Parallelism of tight-binding molecular dynamics simulations is presented by means of the order-N electronic structure theory with the Wannier states, recently developed [J. Phys. Soc. Jpn. 69 (2000) 3773]. An application is tested for silicon nanocrystals of more than millions atoms with the transferable tight-binding Hamiltonian. The efficiency of parallelism is perfect, 98.8%, and the method is the most suitable to parallel computation. The elapse time for a system of 2 � 10 6 atoms is 3.0 min by a computer system of 64 processors of SGI Origin 3800.The calculated results are in good agreement with the results of the exact diagonalization, with an error of 2% for the lattice constant and errors less than 10% for elastic constants.

A new ferromagnetic material excluding transition metals: CaAs in a distorted zinc‐blende structure

A new ferromagnetic material which does not contain transition metals have been theoretically designed. CaAs in a distorted zinc‐blende structure is shown to be ferromagnetic by the first‐principles calculation. A tetragonally compressed structure is an energy minimal and has a total magnetic moment of 1 μB per the chemical formula, although a tetragonally expanded energy‐minimum structure loses the moment. A guideline to fabricate the ferromagnetic material is discussed.

Spin-Polarized Surface States of Metastable Body-Centered Cubic Co(001)

The growth and differential conductivity (d I/d V) spectrum of metastable body-centered cubic (bcc) Co(001) film were studied at room temperature (RT) using scanning tunneling microscopy (STM). Ultrathin (3 ML) epitaxial Co film was grown at RT on clean Cr(001) surface. The bcc structure of the Co(001) film was confirmed from combined low-energy electron diffraction (LEED) and STM measurements. The d I/d V spectrum of the bcc Co(001) film shows a distinct peak at 150 mV. The origin of this peak is identified as a highly spin-polarized minority-spin surface state with a dz2 orbital character as confirmed by our first-principle spin-dependent calculation.

High Pressure Phases and the Structural Phase Transition of Selenium

We studied the structural phase transition of Se under high pressure by means of a full-potential linearized-augmented-plane-wave method. We calculated the electronic structure and the total energy at various volumes for the β-Po type rhombohedral and the bcc structure, from which we estimated the transition pressure. It was found that a first-order phase transition appears at a much lower pressure than the observed value with a small volume reduction. The structural properties calculated were discussed and compared with the available experimental data.

Origin of the simple modulated structures and the pressure induced superconductivity

We have studied origins of the simple modulations of structures using first-principles calculations for group V, VI, and VII elements which have simple modulated structures. For the approximate structures which are defined by removing the modulations, we calculated phonon frequencies along the direction of the wave vectors of the modulation, and searched for possible phonon modes which may relate with the structural modulations. In phosphorus the Madelung energy works to destabilize the approximate monoclinic structure as well as the simple cubic and simple hexagonal structures, while it works to stabilize the approximate structures in other elements. Observing that the SC phase of phosphorus is stabilized by the band energies, we estimated the superconducting Tc in the SC phase. The calculated Tc remains at nearly same values in the SC phase. The electron phonon coupling increases with increasing pressure but the averaged phonon frequency decreases with increasing pressure, which is due to the increasing destabilizing effect of the Madelung energy.

The lattice distortion effect for zinc-blende CrAs and CrSb

We investigated the stability of the ferromagnetism of CrAs and CrSb in the zinc-blende structure against the lattice distortion, systematically. A calculation within the generalized gradient approximation using a full potential linearized augmented plane wave method was performed. We compared the ferromagnetic state and the antiferromagnetic state assuming tetragonal distortion with the lattice constants a and c changing independently and determined the spin polarization ratio in the ferromagnetic phase. The result shows that complete spin polarization (half-metallic ferromagnetism) remains stable even in the presence of large tetragonal distortion. On the other hand, our calculation shows that two monolayers of CrAs is enough to produce a half-metallic state in the CrAs/GaAs multilayer. Thus, the present result suggests that the half-metallic nature persists in various atomic-scale superlattices made of distorted CrAs or CrSb.

Design of new ferromagnetic materials with high spin moments by first-principles calculation

We have searched for a new highly spin-polarized ferromagnet which has a higher spin moment than that of known half-metallic transition metal pnictides with the zinc-blende structure by first-principles calculations. To generate the high spin moment we focus on Gd compounds. Our calculation shows that a (GdN) 1 /(CrAs) 1 structure is a ferromagnetic material. The total magnetic moment of this ferromagnet is over 9.9 μ B per chemical formula.

Electronic structure and structural stability of the high-pressure orthorhombic phase of selenium

Electronic and atomic structures for high-pressure orthorhombic selenium were studied with first-principles calculations. Structural optimizations for four atoms in the orthorhombic unit cell were performed and the space group of the optimized structure was determined as P21/m, which is a monoclinic space group. The atomic structure optimized and the space group determined are consistent with the result of the x-ray diffraction measurement. The electronic density of states of the optimized structure is quite similar to that of the structure in the next high-pressure phase. This fact corresponds well to the experimental result that the transition to the next phase is a second-order one.