Theoretical Studies on Electronic State and Superconductivity in Oxide Superconductors

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The superconductivity in the oxide superconductors appears only in the metallic region close to the metalinsulator transition, and to clarify the properties of electronic states around this region is considered one as of the keys to understand the mechanism of the high-T_c superconductivity. We calculated the electronic density of states selfconsistently by use of the d-p mixing model among Cu-orbits and O-orbits, including many body effects induced by spin and charge fluctuations. We have found that new states are formed inside the charge transfer insulator gap with carrier doping due to many body effects. The obtained results explain qualitatively the experimental results of spectral intensity of photoemission and optical conductivity. We published these results in 14 papers. The theoretically obtained new states at the Fermi level are expected to include large charge fluctuation. A possibility for such fluctuation to work as an attractive force in the mechanism of the superconductiv … More ity is under investigation at present.Due to their layered structures, cuprate oxide superconductors also show various anomalous electronic and magnetic properties in the superconducting state. We have proposed the model in which the high-T_c superconductivity appears primarily in the CuO_2 planes by certain attractive force and superconductivity on the other layers is induced by the proximity effect. From this model, we calculated tunneling conductance, anisotropy of the optical conductivity, Knight shift and nuclear magnetic relaxation rates of ions on various layers. Variety in appearance of physical quantities in the experimental observation can be understood in a unified way by use of our model. Structure of flux line in the above mentioned model are expected to be very anisotropic. We predicted the intrinsic pinning effect and anomalous behaviors of magnetization curves. These results are published in 14 papers. A part of the above results was presented in the invited talks at the 19-th international conference of low temperature physics in Brighton, England. Less