Investigation of the Electronic Structure of High-Temperature Superconductors by Polarized X-Ray Absorption Spectroscopy

Abstract

The electronic structure of high-temperature superconductors near the Fermi level which is of primary interest for the understanding of the mechanism of superconductivity, is not yet well understood. Most theoretical investigations deal with Cu3d_x−y and O2p_x,y orbitals at E_F, but whether or not O2p_z and Cu3d_3z−r orbitals contribute to states at E_F, is still under discussion. The contribution of states with out-of-plane orbital character has been investigated by polarized x-ray absorption spectroscopy (XAS) on HTSC single crystals. For La_2−xSr_xCuO₄ a small contribution from O2p_z orbitals (8% of holes in O2p valence band states) was observed which increased to about 13% on 30% Sr doping. In n-type doped HTSC there is no clear evidence for O2p_z orbitals at E_F, but states are observed slightly above E_F with O2p_z character depending on the doping and annealing conditions. An observed admixture of Cu 3d_3z−r states to the predominant 3d_x hole states at E_F of a few percent did not significantly exceed the expermental error limits.