METAL-INSULATOR-TRANSITION IN OXIDE SUPERCONDUCTORS

Change of electronic states near the metal-insulator transition in copper oxide superconductors is discussed by the use of the p-d mixing model in the CuO2 plane. The strong intra-atomic Coulomb repulsion at the Cu-sites induces the Hubbard splitting in d-levels and causes the strong correlation among p-electrons and Cu-spins in the process of the p-d hopping. Results show that a tail structure of p-electron density of states extending toward the Fermi level is induced even in the insulator phase and that by hole doping new narrow states develop at the Fermi level. It is shown that the similar results are obtained from the Kondo-type model reduced from the p-d mixing model. The results seem to explain qualitatively the experimental features observed in EELS, photoemission, XAS and optical conductivity.