ELECTRONIC VALENCE BAND-STRUCTURE OF HIGH-TC SUPERCONDUCTORS - X-RAY-EMISSION SPECTROSCOPY STUDY

The distribution of the electronic density of O 2p and Cu 3d states in a series of bismuthates and cuprates has been studied by X-ray emission spectroscopy (XES). It has been found that in BaPb1-xBixO3, La2-xSrxCuO4 and YBa2Cu3O7-x, a high-energy shift of the O K-alpha spectrum in both photon energy and Fermi level-related scales occurs during the transition from semiconducting to superconducting phase. This shift is explained by the redistribution of 2p states of apical oxygen atoms. The binding energy of Cu 3d states for YBa2Cu3O6.2 is rather low in contrast to that for YBa2Cu3O6.9 because of the presence of monovalent copper atoms in the semiconductor as a result of oxygen depletion of Cu-O chains. A correlation between T(c) and the energy localization of the O 2p band relative to E(F) has been observed in a series of cuprates, the superconducting transition temperature of which varies from 35 to 125 K. On increasing T(c), O 2p states shift towards the Fermi level while the binding energy of Cu 3d states insignificantly rises.