Yoshimasa Murayama

Micromagnetics on Stripe Domain Films. : I. Critical Cases

A micromagnetic calculation was made in order to estimate the width of the stripe domains in a magnetic thin film with a perpendicular anisotropy, which drives the magnetization out of the plane. The rotation of spins from the average magnetization, not only out of plane (similar to the µ * correction) but also in the plane (similar lo the closure domain), was taken into consideration. The rotating angles in both senses were assumed to be functions of the depth from the film surface and of the position across the stripe domain. They were determined so as to satisfy a set of Eulers equations, and solved numerically and also semi-analytically. The results are in good agreement with the experiments by Saito et al. .

A two-dimensional device simulator of semiconductor lasers

Abstract A two-dimensional simulator for aid in designing semiconductor lasers is developed. Poissons equation and the current continuity equations for electrons and holes as well as the wave equation and rate equation for photons are numerically solved. Heterojunctions and carrier degeneracy are rigorously treated, and analytical results on channeled-substrate-planar lasers are presented to demonstrate the simulator. Reasonable agreement is found between calculated and experimental results, and calculated results clarify precisely the operation mechanism of semiconductor lasers. The present work enables computer simulation for the first time to be a practical design aid in research and development of various kinds of semiconductor lasers.

Landau-Level Broadening in GaAs/AlGaAs Heterojunctions

The broadening of Landau levels is calculated in a two-dimensional electron system at a GaAs/AlGaAs heterostructure in strong magnetic fields. Scatterers are assumed to be charged ions in the AlGaAs layer and in the bulk GaAs layer. The self-consistent determination of the broadening and screening is shown to be essential and gives rise to a drastic reduction of the broadening when the spacer thickness is sufficiently large. The broadening of cyclotron resonance and the peak values of the diagonal conductivity are also calculated.

Micromagnetics on Stripe Domain Films. II

It was studied theoretically whether a periodic modulation of the spin configuration along the extended stripes of the stripe domain walls lowers the total energy or not. So long as the films of critical thickness are concerned, the asymptotic solution shows that the modulation of that kind is not favored from the view point of the optimal energy.

Mean Field Theory of the Holon-Spinon System. I

Mean field approximation is applied to nonlinear equations of motion in the holon-spinon operator formalism proposed previously. A simple interpolation scheme, which looks very much like the conventional band theory, is thus established between insulating and metallic limits of the tJ model. The transport equation of the normal state is given and critical temperatures of S -wave and D -wave superconducting states are calculated.

The Mechanism of Hole-Doped High-Tc Superconductivity

By applying the holon-doublon formalism to a highly correlated system consisting of d - and p -electrons, a theory is developed concerning whether a superconductive phase can be realized in parallel to the previously investigated single-band model. The result follows: those d -states hybridized with (partially occupied) p σ+ -band form pairs and are condensed into superconductive ground state. On this model almost all features characteristic to hole-doped, layer-structured high T c superconductors are, at least qualitatively, interpreted.

Macroscopic quantum tunneling in thin superconducting wires

Abstract Phase slippage in a thin superconducting wire due to macroscopic quantum tunneling is discussed within the framework of a phenomenological time-dependent Ginzburg-Landau equation near the absolute zero of temperature. The theory predicts fluctuation-induced smearing of the voltage step at the critical current. Voltages generated by phase slippage are shown to be nonlinearly dependent on current and to be comparable to those due to thermal activations near the critical temperature.

Electronic band calculations for zinc-blende BN with nonlocal density functionals

Abstract We have examined the ground-state and bands properties of zinc-blende BN using a pseudopotential approach with the nonlocal exchange potential proposed by Perdew and Wang (PW). We find that the PW potential improves the cohesive energy and the band gap compared with local density (LD) approximation.

Power-law resistive behavior of two-dimensional Josephson junction array with bond defects: Dynamic simulation and its relevance to high-Tc oxides

Abstract Nonlinear electric resistivity of the two-dimensional(2D) Josephson junction array with bond defects (defect density p ) is calculated directly using the Langevin simulation technique. It is found that both the current-voltage ( I-V ) characteristics and the p-V resistive behavior at low temperatures obey power-laws with continuously varying exponents: V ∝ I α p and V ∝ p β I . Computational results are quite similar to those observed experimentally in the ion irradiation effect on high- T c oxides. The ion irradiation is thought to create random defects in the 2D intrinsic weak-link structure which is peculiar to the high- T c oxide superconductor. It is suggested that the vortex-antivortex pair excitation and its transport property in 2D media with bond disorder are crucial for the defect-dependent power-law.

Effect of dissipation on macroscopic quantum tunneling in thin superconducting wires

Abstract Macroscopic quantum tunnelings in thin superconducting wires are discussed within the framework of the time-dependent Ginzburg-Landau equation. The dissipation is described by introducing the reservoir oscillators of infinite degrees of freedom and eliminating them in the path integral representation of the partition function. The instantaneous Coulomb interaction is shown to maintain the charge neutrality due to a kind of Debye shielding in superconductors, according to which the formulation of the tunneling probability is greatly simplified. The derived temperature dependence of the tunneling rate is compatible with recently observed phenomena in thin indium wires near the critical temperature.