UNOCCUPIED ELECTRONIC-STRUCTURE OF SR2CUO2CL2 AND BA2CU3O4CL2 - EXPERIMENT AND THEORY

The unoccupied electronic structure of the layered cuprates Sr2CuO2Cl2 and Ba2Cu3O4Cl2 has been studied using polarization-dependent x-ray a bsorption spectroscopy at the O 1s and Cu 2p(3/2) edges and band-struc ture calculations within the local-density approximation. In contrast to almost all high-temperature superconductors, there are no oxygen at oms outside the Cu-O planes in these oxychlorides. Our results represe nt therefore direct experimental information regarding the oxygen-deri ved unoccupied electronic structure of undoped Cu-O planes in square-p lanar coordination. The O 1s and Cu 2p(3/2) x-ray absorption spectra o f Sr2CuO2Cl2 and Ba2Cu3O44Cl2 are quite similar with an essentially tw o-dimensional upper Hubbard band and extra oxygen and copper related a bsorption fine structure at higher energies. From its polarization dep endence and a comparison to the results of the band-structure calculat ions, it is concluded that this extra fine structure is related to tra nsitions into O 2p and Cu 3d3d(3z2-r2) orbitals which have become part ly unoccupied due to hybridization with Sr 4d/Ba 5d/Cu 4p(z) and Cu 4s orbitals, respectively. Differences between these two compounds are t he larger width of the upper Hubbard band and the higher relative Cu 3 d(3z2-r2) hole occupation above the upper Hubbard band in Ba2Cu3O4Cl2. These differences can be related to the extra Cu-B atoms in the Cu3O4 planes of Ba2Cu3O4Cl2 with respect to the CuO2 planes of Sr2CuO2-Cl-2 . [S0163-1829(98)03706-0].