O. Strebel

Valency and covalency in high-Tc-superconductors from x-ray absorption

Abstract The local electronic structure of the high-Tc superconductors YBa2Cu3O7−δ and Nd2−xCexCuO4−δ is probed by soft x-ray absorption spectroscopy at the Cu LII,III, O K, Ce MIV,V, and Nd MIV,V thresholds. The near-edge structures provide information on the partial unoccupied density of states above EF as well as on valency and covalency of the constituent elements.

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

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 Cu3dx−y and O2px,y orbitals at EF, but whether or not O2pz and Cu3d3z−r orbitals contribute to states at EF, 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 La2−xSrxCuO4 a small contribution from O2pz 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 O2pz orbitals at EF, but states are observed slightly above EF with O2pz character depending on the doping and annealing conditions. An observed admixture of Cu 3d3z−r states to the predominant 3dx hole states at EF of a few percent did not significantly exceed the expermental error limits.

INTERFACE FORMATION BETWEEN METALS AND HIGH-TC SUPERCONDUCTORS

ABSTRACT The results of Photoemission studies using synchrotron radiation on the chemical and electronic structure of interface between ceramic superconductors (YBa2Cu3O7−δ, Bi5Sr3Ca2Cu4Oy) and metals (Ag, Rb, Mg) are presented. The interfaces with Ag are non-reactive, with no noticeable changes in the stoichiometry of the superconductor at the interface. On the other hand, Mg and Rb reduce the ceramic superconductors inducing a metal-semiconductor transition in the interfacial region.