S. Takahashi

Thermopower in Ce Kondo systems

The giant thermopower in metals with Ce 3+ impurities is theoretically studied. The relaxation time of a conduction electron is calculated as a function of energy in the selfconsistent ladder approximation by applying Abrikosovs pseudofermion technique to the Coqblin-Schrieffer exchange model with crystal field. The thermopower as well as the electrical resistivity exhibit universal behaviors if the crystal field is neglected. The crystal field induces the characteristic features and the anisotropy in the thermopower versus temperature curve. However, the sign of the thermopower is always positive in Ce system in contrast with Yb systems.

Effect of Crystal Field on Kondo Resistivity in Ce Compounds

The effect of crystal field on Kondo resistivity in metals with Ce impurities is theoretically studied. The T -matrix of conduction electron is calculated in the self-consistent ladder approximation by applying Abrikosovs pseudofermion technique to the Coqblin-Schrieffer exchange model. The numerical results of the resistivity in cubic and tetragonal crystals are compared with the experimental ones in Ce x La 1- x Al 2 and CeCu 2 Si 2 . Our theory provides a unified view of the magnetic and transport properties in Ce compounds and alloys.

Crystal field and Kondo effect in Ce compounds

Abstract The static and dynamical susceptibilities in Ce compounds and alloys are studied theoretically, taking into account Kondo effects and crystal fields self-consistently. A quantitative comparison of the neutron scattering spectra is made with the experimental data in CeCu 2 Si 2 , Ce x La 1− x Al 2 and CeB 6 .

Kondo effect on crystal field splitting

A theory is developed for the temperature dependence of crystal field energy levels of Ce ions in metals caused by the Kondo effect. Abrikosov’s pseudofermion technique is applied to the Coqblin–Schrieffer exchange model, and the self energy is derived in the self‐consistent ladder approximation. The neutron quasi‐elastic and inelastic scattering spectra are calculated in the system with cubic crystal field.

Kondo-like Effect of Atomic Motion in Amorphous Superconductors

Effects of tunneling motion of atoms characteristic in glass systems on superconductivity is theoretically studied. The pair-breaking mechanism of superconductivity proportional to [ln ( T / D )] 2 is derived in the fourth order of the Coulomb interaction between conduction electrons and atoms, where T is the temperature and D is the band width. We propose that the pair-breaking mechanism is an origin of the universal absence of the Maki term in the paraconductivity in amorphous superconductors.

Magnetization of thin films of ferromagnetic superconductors

Abstract The magnetization of thin films of ferromagnetic superconductors is theoretically studied. The symmetric and antisymmetric magnetic orders appear in the films depending on the thickness of the films. The magnetization for the symmetric order monotonically increases with decreasing temperature. However, the magnetization curves for the antisymmetric order has a sharp peak near the magnetic phase transition temperature in a weak external magnetic field.

Magnetic and transport properties in Ce compounds: Kondo effect and crystal field

Abstract We present a theory for the magnetic and transport properties in metals with Ce impurities, which takes into account the Kondo effect and the crystal field self-consistently. The theoretical results are compared with the experimental data in CeCu 2 Si 2 .

Thin Films of Ferromagnetic Superconductors

The effect of the electromagnetic interaction between the persistent current and the rare earth moments is theoretically studied in the film of ferromagnetic superconductors. Depending on the film thickness and temperature, the ferromagnetic-like order, the spin-spiral-like order and the spin-sinusoidal-like order are predicted to occur in coexistence with superconductivity. From the calculation of the free energies of these states and the normal ferromagnetic state, the phase diagram is obtained in the plane of temperature and film thickness.

Films and surfaces of ferromagnetic superconductors

Abstract The ternary compounds such as (RE)Rh 4 B 4 and (RE)Mo 6 S 8 contain magnetic rare earth ions RE at the regular lattice sites, nevertheless they show superconductivity. In the superconducting state of films and surfaces of these compounds, the interplay between magnetism and superconductivity causes the following unusual phenomena: the magnetization bound within the London penetration depth from the surface appears below a critical temperature. In the films various types of the magnetic orders coexisting with superconductivity appears depending on the film thickness. The anomalous behavior of the Josephson effect in ferromagnetic superconductors is briefly discussed.

Kondo Effect versus Crystal Field

In Ce compounds and alloys, the scattering of conduction electrons by Ce3+ ions is accompanied by transition between the crystal field energy levels. We present a theory for the magnetic and transport properties which takes into account the Kondo effect and the crystal field self-consistently. Through the analyses of the magnetic susceptibility, the neutron quasi-elastic and inelastic scattering spectra, and the resistivity, we propose the crystal field level-scheme in CeCu2Si2.