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A highly correlated [ital p]-[ital d] model of cuprate oxide superconductors is analyzed by use of composite operators on CuO[sub 2] clusters. A minimal set of composite operators to describe the electronic state is identified from the criterion that the order of the approximation is determined by coupling constants multiplied by the weight of their respective operators. It is shown that the intensity transfer among bands with hole doping is well described within the mean-field approximation. Effects of the nearest-neighbor spin correlation are investigated and it is shown that the band dispersions are strongly affected. By the local antiferromagnetic correlation, the band dispersion of the upper Hubbard band is flattened in the whole Brillouin zone, while that of the [ital p] dominant band near the Fermi level is flattened only around a certain region centered at the zone boundary. In the moderately doped region, the Fermi surface is consistent with a large Fermi surface.

The vortex structure and the distribution of the transport current in a layered superconductor consisting of an alternate stack of strongly and weakly superconducting layers are theoretically studied within the framework of the Ginzburg-Landau theory. It is assumed that in the superconductor the coherence length is comparable to the layer thickness and the penetration depth is much longer than the thickness. The present theory predicts that the superconducting current mainly flows in the strongly superconducting layer, while the vortex center lies in the weakly superconducting layer. As a result the driving force acting on the vortex becomes extremely weak, since the small current density in the weakly superconducting layer is responsible for the force

Pr 3d and 4d core levels have been studied for ${\mathrm{Y}}_{0.4}$${\mathrm{Pr}}_{0.6}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8}$ and ${\mathrm{PrBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ using x-ray-photoelectron spectroscopy to elucidate the valence states. The features of Pr 3d and 4d are close to those for ${\mathrm{Pr}}_{2}$${\mathrm{O}}_{3}$ for both compounds, while they are different from those for ${\mathrm{Pr}}_{6}$${\mathrm{O}}_{11}$. It is concluded that praseodymium is almost trivalent with a 4${\mathit{f}}^{2}$ configuration.

A formulation of the p-d-mixing model of oxide superconductors is presented by introducing composite operators that describe electronic excitations associated with the Cu-O covalent bond. A conduction band developing inside the charge-transfer gap with hole doping can be understood as the band formed by mixing among p and d electrons and these composite electronic excitations

A formulation of a self-consistent perturbation method is presented, which is convenient to deal with the strongly correlated p-d mixing model. A real-time formulation of thermofield dynamics is used. The method allows one to develop a finite-temperature perturbation theory for operators that do not allow the use of Wick's theorem. By introducing local decomposition of the operators, a diagram method that does not rely on Wick's theorem is constructed. For the p-d mixing model, self-consistent equations for the electron Green's functions are derived.

Electronic states near the metal-insulator transition in oxide superconductors are studied in the p-d mixing model by use of the Green's-function method. Due to spin and charge fluctuations induced in the p-d hopping, electronic states near the Fermi level are strongly renormalized and show anomalous behavior in the insulator-to-metal transition; that is, the occupied p band has a tail structure at the top of the band and with hole doping, an additional narrow band develops at the Fermi level inside the charge-transfer gap, splitting from the main p band. The main p band gradually approaches the Fermi level with hole doping. The results seem to explain qualitative behaviors of experiments in photoemission and optical conductivity.

The upper critical field H/sub c/2 in the Kondo lattice was theoretically studied in connection with superconductivity in CeCu/sub 2/Si/sub 2/ and UBe/sub 13/. To describe the coherent state in the Kondo lattice, a periodic Anderson model with multiple conduction bands was used. With the use of a single-site approximation for the self-energy of the f electron, a Fermi-liquid state was derived in the presence of external magnetic field. By assuming a singlet superconductivity of the Fermi liquid, H/sub c/2 was calculated as a function of temperature and the scattering potentials due to impurities. In comparison with the experiments in CeCu/sub 2/Si/sub 2/ and UBe/sub 13/, the nature of superconductivity in these compounds: that is, singlet or triplet superconductivity: was discussed.

The upper critical field of superconducting superlattices is calculated by taking account of spatial variations of the following quantities of conduction electrons: the density of states, the diffusion constant, the attractive interaction constant responsible for superconductivity, and the spin polarization. For applied magnetic fields parallel to the layers, these spatial variations cause a dimensional crossover in the temperature dependence of the upper critical field ${H}_{c2?}$. The spatial variation of the density of states is most important for the effect of the dimensional crossover observed in Nb/Cu superlattices. For applied magnetic fields perpendicular to layers, a new type of crossover is predicted in the temperature dependence of ${H}_{c2\ensuremath{\perp}}$ for superlattices composed of a high-${H}_{c2}$ material and a low-${H}_{c2}$ material. The anisotropic spin polarization of conduction electrons induced by applied magnetic fields explains the anomalous behavior of ${H}_{c2}$ observed in V/Ni superlattices.