Yoshimasa Isawa

Electron correlation and excitation spectra in high-Tc Cu oxides

Abstract Taking into account the electron correlation on Cu-sites and the mixing interaction between d-holes on Cu-sites and p-holes on O-sites, the electronic states in high- T c Cu oxides are investigated by using the operator transformation method. Singlet correlation between holes on Cu- and O-sites proposed by Zhang and Rice results in charged bosons and fermions of spin degrees of freedom. Such separation of degrees of freedom is not expected in another strongly correlated system, i.e. the so-called heavy fermions. We examine the RVB superconductivity and the excitation spectra in the Cu oxides.

Far-Infrared Absorption to Test Anisotropy of Energy Gap in High Tc Superconducting Oxides

The far-infrared absorption spectra are theoretically studied in poly-crystalline superconductors with anisotropic energy gap. The discrepancy between the recent far-infrared and tunneling experiments in the high Tc oxides, (La, Sr)2CuO4, are explained by the anisotropy of the energy gap.

Far-Infrared Absorption and Tunneling Conductance of Anisotropic Singlet Superconductors at Finite Temperatures

The far-infrared absorption spectra and the quasi-particle tunneling conductance are calculated at finite temperatures in poly-crystalline samples of superconductors with anisotropic singlet pairing. Currently observed small 2Δ/kBTc ratio in far-infrared measurements and the large ratio in tunneling measurements for high-Tc oxides are shown to be consistent with the anisotropic pairing.

Excitation spectra and bose condensation in superconductors with strong correlation

We study the pairing mechanism of high Tc oxides based on the Hubbard model. The transformation of the electron operators to a set of four operators is introduced for the analysis. We discuss the condensation of the resonating valence bond and the Bose condensation responsible for the superconductivity together with the excitation spectra. The effects of the long range intersite repulsive interaction which causes a dramatic change in the hole excitation spectra is also discussed. The formulas for the specific heat at low temperatures and the resistivity in normal state are given.

Effects of Magnetic and Non-Magnetic Impurities on Anisotropic Singlet Superconductors

We examine effects of magnetic and non-magnetic impurities on superconductors with anisotropic Cooper pairs of spin-singlet and calculate the tunneling density of states. When the Cooper pair has no component with s-symmetry, magnetic impurities result in the same pair-breaking effect as non-magnetic ones, which is weaker than the usual time-reversal symmetry breaking. The recently discovered high Tc magnetic superconducting oxides, R-Ba-Cu-O with R being rare-earth ions, are discussed in the light of the present theory.

Coherent phenomena in small systems

Abstract We develop a new theoretical method of treating the small system in a weakly localized regime. By applying it to short metallic lines, we predict that the periodic flux oscillations of the magnetoresistance could be found in singly connected systems, which shows that a sample geometry of doubly connected shape is not required for such oscillations. Our conclusion is that these oscillations are the result of diffraction effects in the magnetic field caused by the interference of many coherent waves in small systems.

Gap structure and spin excitation in high-Tc oxides

Abstract Antiferromagnetic dynamical spin susceptibility χ 0 is calculated in the superconducting state of the two-dimensional square lattice. Assuming d-wave pairing and taking the repulsive on-site Coulomb interaction through RPA, one obtains a spin mode of ω∼2Δ 0 and q ∼ Q where Δ 0 is the maximum gap and Q is the nesting vector of tight-binding half-filled band.

Superconducting Energy Gap and Pairing Interaction in High Tc Oxides

We propose two characteristic features in high Tc oxides, La-Sr-Cu-0 and Y-Ba-Cu-O: (i) The apparent discrepancy of the superconducting energy gap between far-infrared and quasi- particle tunneling measurements can be an indication of anisotropic Cooper pairing with spin singlet. (ii) The Coulomb interaction induces spin wave mode with the wave number of the Brillouin zone boundary and the energy close to 2Δ0, Δ0 being the maximum value of the superconducting energy gap. This mode may be observed by neutron inelastic scattering and tunneling experiments.

Phase coherence, nonlocal conductivity and A-B effect in normal metals

The validity of the basic formula for the voltage fluctuation in network systems, expressed in terms of the quantum correction of the non-local couductivity, is discussed. The characteristic features of the Aharonov-Bohm oscillation in such a system with a ring, where no transport current flows, are presented.

Metallic impurity conduction and metal-nonmetal transition in n-InSb at low temperature

Abstract The resistivities of n-InSb in the metallic impurity conduction range were investigated down to 20 mK at zero and 100 Oe of applied magnetic field. Samples in the metallic impurity conduction range were divided into two groups. The resistivities of samples with high carrier concentration take maximum values at several hundreds mK. Below this temperature, the resistivities decrease when the temperature is lowered. On the other hand, the resistivities of samples with low carrier concentration increase when the temperature is lowered. We found that the dominant mechanisms which determine the sign of the temperature coefficient of the resistivities at zero magnetic field are different from each other in the two groups of the samples.