ORBITALS, CORRELATION AND VALENCIES IN HIGH-TC SUPERCONDUCTORS

A survey of certain properties of high-T(c) superconductors is given in connection with details of their electronic structure such as the kind of orbitals involved and the degree of correlation. Special attention is given to the properties of cuprates at high doping level. The question of the existence of a 'Mott transition' at high electron or hole concentration is addressed. We also discuss physical factors (d-p Coulomb interaction, orbital mixing) leading to the partial occupation of the copper d(z)2 orbital. In particular, we show that in the localized picture (x2 - y2) and z2 levels in La(2-x)Sr(x)CuO4 may cross at x almost-equal-to 0.4. This crossing may be responsible for a marked change of many properties at this doping. The possible role of the z2 electrons in pairing is discussed in connection with some recent experiments.