Masami Ashida

Order-Disorder and Displacive Transitions in a Quantum Ising Model

A ferrodistortive phase transition is analyzed for a quantum particles in a simple quartic potential. The interactions are taken into account by the mean-field approximation. A quantum thermodynamic treatment gives analytic expressions for the static susceptibility, the specific heat and the soft mode frequency. The Rhodes–Wohlfarth ratio is a measure whether the system is order–disorder or displacive, however, the border is fuzzy depending on the interaction strength. The quantum effect is discussed when the transition takes place at low temperature. Whether the atomic density is a single or multiple peak distributions seems to be a definite criterion for the character of the structural phase transition.

Propagation of zero sound in superfluid3He-B under magnetic field

We present a theory of zero sound propagation in super fluid3He-B with an order parameter strongly distorted by magnetic field. A general formula of the dispersion relation for arbitrary magnetic field in the collisionless regime is derived within the weak coupling theory and under the assumption of particle-hole symmetry. The Landau parameters are taken into account up toF2s andF0a. Numerical results for the sound velocity and absorption spectrum are presented. We show that the collective modeJ = 1,Jz = 0 yields a sizable peak in the sound absorption spectrum under weak but finite magnetic field. The quasi-particle excitations under magnetic field also exhibit cusp-like fine structures in the absorption spectrum. We show that the anomalies discovered by Ling et al. and Sounders et al. near the pair breaking edge in theq ⊥H geometry consist of theJ = 1,Jz = 0 collective mode and the pair breaking cusps in theJz = 0, ±2 channels.

Quantum Susceptibility for Structural Transitions Based on Exact Solutions of the Schrödinger Equation

The static susceptibility of a quantum system is considered to discuss the structural phase transition. The linear response theory is applied with assuming a mean field treatment for the interaction between unit cells. The energy eigen state is solved exactly for the Schrödinger equation of a single particle within a two-Morse type self-potential. The parameter dependence of the transition temperature is discussed analytically, which will give a general insight into the structural phase transition. The pressure-temperature phase diagram for KDP-type crystal is constructed theoretically.

Effect of Finite Reflection at the Interface on Diamagnetic Response of Normal-Superconducting Proximity Contact Double Layers

A quasi-classical theory of diamagnetic response of normal-superconducting proximity contact double layers with a finite reflection at the interface is developed for arbitrary concentrations of impurities. It is found that the reflection at the interface has strong influence on the screening fraction of a system with a finite concentration of impurities, as is the case in the clean limit. Taking into account the finite reflection at the interface, we calculate the temperature dependence of the screening fraction and find good agreement with the recent experimental results obtained for Cu/Ta and Ag/Nb samples.

Superconducting Transition Temperature of Superconducting-Normal Periodic Multi-Layer

Transition temperature T c of superconducting-normal (S-N) periodic multi-layer system is studied in the clean limit. The finite reflection coefficient R at the interface is explicitly taken into account. The obtained gap equation of S-N multi-layer is rather compact and is similar to that of S-N double-layer. The behavior of T c of a periodic multi-layer qualitatively agrees with that of the corresponding double-layer except when the thickness of layer is shorter than the coherence length and R ≃0.5 . Thickness dependence of T c of a periodic multi-layer is in good agreement with the experimental data of Banerjee et al. on the Nb/Cu superlattice.