Masafumi Shirai

Material Design of Half-Metallic Zinc-Blende CrAs and the Synthesis by Molecular-Beam Epitaxy

A new class of half-metallic ferromagnets has been found in the zinc-blende crystal structure. The previously nonexistent zinc-blende CrAs thin films have been synthesized on GaAs (001) substrates by molecular-beam epitaxy, and show a ferromagnetic behavior at room temperature. The zinc-blende CrAs has been designed by ab initio calculations based on the local spin-density approximation, and the calculation predicts the highly spin-polarized electronic band structure.

Band structures of zinc-blende-type MnAs and (MnAs)1(GaAs)1 superlattice

Abstract Electronic band-structures of the hypothetical zinc-blende phase of MnAs and the (MnAs) 1 (GaAs) 1 (0 0 1) superlattice are calculated by using the full-potential linearized augmented-plane-wave method in order to discuss the effect of Mn-substitution for Ga in GaAs on their electronic and magnetic properties. The magnetic polarization antiparallel to the Mn moment is induced at the As site in the ferromagnetic state of both systems, which indicates the antiferromagnetic coupling between Mn-spins and valence-band carriers having mainly As 4p character.

Epitaxial growth of zinc-blende CrAs/GaAs multilayer

Zinc-blende CrAs/GaAs multilayers were grown by molecular beam epitaxy. It was certified that each CrAs layer maintains an epitaxial relationship with the zinc-blende GaAs structure judging from the reflection high-energy electron diffraction observation. The film contains thicker zinc- blende CrAs layers in total than the CrAs thin film directly grown on the GaAs substrate which has the critical thickness of 3 nm. It was clarified that the optimum thicknesses of CrAs and GaAs to keep a good epitaxial relationship are 2 ML and 2 ML, respectively. The electronic structure of the multilayer is thought to be close to that of the (Ga, Cr)As thin film which has 50% of Cr content judging from x-ray absorption spectroscopy measurements.

Electron−lattice interaction and superconductivity in BaPb1-xBixO3 and BaxK1-xBiO3

The electron-lattice interaction of BaPb1-xBixO3 (BPB) and BaxK1-xBiO3 (BKB) is studied microscopically by using the realistic electronic bands of BaBiO3 reproduced by the tight-binding model. It is found that the electron-lattice coupling coefficients have strong wavevector and mode dependences. The electron-lattice interaction causes a remarkable renormalization of the longitudinal oxygen stretching and/or breathing mode vibration. Superconductivity is discussed in the framework of the strong-coupling theory of the phonon mechanism. The spectral function alpha 2F( omega ) has some prominent structures in the frequency range of the oxygen stretching/breathing mode. As x increases, some of the main peaks in alpha 2F( omega ) shift to the lower-frequency side, reflecting the phonon frequency renormalisation. The transition temperature Tc and the energy gap function Delta ( epsilon ) at T=0 K have been evaluated by solving the Eliashberg equations. The calculated Tc increases rapidly with increasing x, and reaches about 30 K for x=0.7. The oxygen isotope shift of Tc in BKB is calculated and the characteristic exponent alpha defined by Tc varies as MOalpha (MO is oxygen atomic mass) is evaluated to be 0.35-0.45. The superconducting energy gap Delta 0 is evaluated to be 4.8 meV for x=0.7. The ratio 2 Delta 0/kBTc is found to have a value close to that predicted by the Bardeen-Cooper-Schrieffer weak-coupling theory (2 Delta 0/kBTc=3.5). However, the calculated tunnelling differential conductance dI/dV and its derivative d2I/dV2 show behaviours that are characteristic to the strong-coupling superconductor.

Electronic and magnetic properties of 3d transition-metal-doped GaAs

Abstract Electronic band-structure calculations are carried out for the hypothetical zinc-blende phase of 3d transition-metal monoarsenides as well as periodic supercells of 3d transition-metal-doped GaAs by using the full-potential linearized augmented-plane-wave method. The antiferromagnetic state is stable for zinc-blende FeAs, while the ferromagnetic state is stable for zinc-blende VAs, CrAs, and MnAs. It is expected that (Ga,Cr)As becomes ferromagnetic due to the presence of mobile valence-band carriers (holes), since the electronic band-structure in the ferromagnetic state of (Ga,Cr)As is very similar to that of (Ga,Mn)As.

Epitaxial growth of new half-metallic ferromagnet zinc-blende CrAs and the substrate temperature dependence

Abstract Epitaxial zinc-blende CrAs thin films were grown at two different temperatures. CrAs (2xa0nm) grown at 200°C formed plateau-shapes, whereas CrAs (2xa0nm) grown at 300°C formed dispersed dots. The thin film grown at 200°C showed ferromagnetic behavior at room temperature, and the Curie temperature was estimated to be over 400xa0K.

Effect of intercalation on structural instability and superconductivity of layered 2H-type NbSe2 and NbS2

Abstract Effects of intercalation on the lattice instability and the superconductivity of NbSe2 and NbS2 are studied in the framework of the rigid band approximation. In the case of doping acceptor-type intercalants into NbSe2, the point of lattice instability in q-space is moved to q = Γ M from q = 2 3 ΓM . Further, the frequency renormalization is enhanced and the CDW transition temperature is expected to increase. In the case of doping donor-type intercalants, on the other hand, the frequency renormalization is weakened and the lattice instability is suppressed. The same tendency is found also in NbS2. The effect of intercalation on the superconducting transition temperature Tc of NbS2 is investigated by calculating the functional derivative of Tc with respect to the spectral function. The donor-type intercalant lowers Tc as observed in the case of alkali-metal doping. On the other hand, the acceptor-type intercalant raises Tc as long as the lattice instability does not take place.

Electronic band structure and magnetic and optical properties of Fe7Se8 and Co7Se8

Abstract First-principles electronic band structure calculations are performed for cation-deficient NiAs-type transition-metal compounds Fe7Se8 and Co7Se8 by using the LAPW method. The magnetic moment calculated for ferrimagnetic Fe7Se8 agrees well with the observed value. Photoemission and optical conductivity spectra are calculated for both compounds and the results are compared with the observations.

Spin polarized band structure of Cu3Au-type compounds MPt3 (MV, Cr, Mn, Fe, Co)

Abstract First-principles calculation is performed for the spin-polarized band structures of MPt 3 (Mue5f8V, Cr, Mn, Fe, Co) by using a scalar-relativistic LAPW method. Calculated values of the spin moments of M atoms are in good agreement with the observations. The spin moment of Pt atom is antiparallel to that of M atoms for Mue5fbV and Cr, and parallel for Mue5fbMn, Fe and Co.

Electron-phonon interaction, lattice dynamics and superconductivity of an oxide spinel LiTi2O4

The electron-phonon interaction of an oxide spinel LiTi2O4 is calculated on the basis of a realistic electronic band structure, which is obtained by the tight-binding model so as to reproduce the first-principles bands. The lattice dynamics of LiTi2O4 is studied by taking account of the effect of the electron-phonon interaction. Due to the characteristic dependences of the electron-phonon interaction on wavevectors and vibrational modes, a remarkable phonon frequency renormalization is found in the frequency range of 30-80 meV over a wide region of the Brillouin zone. By using the electron-phonon interaction and the renormalized phonon frequencies, the electron-phonon spectral function alpha 2F( omega ) is calculated to provide a basis for understanding the superconductivity of LiTi2O4. The superconducting transition temperature, gap functions, tunneling spectra and other thermodynamic properties are studied by solving the Eliashberg equation. The calculated results are in good agreement with observations.