Site-specific and doping-dependent electronic structure of YBa2Cu~O"probed by 0 1s and Cu 2p I-ray-absorption spectroscopy

A series of high-T, superconductors was studied using soft x-ray absorption spectroscopy at the Cu-Lii, iii thresholds. From a comparison with CuO and NaCu02, it is shown that these superconductors contain mostly 3d9 and 3dsL (L oxygen ligand hole) configurations in the ground state, with negligible amounts of Cu(III) 3ds. The spectral weights of the 3d9L components are found to be higher for the YBa2Cu307-type superconductors than for the La2Cu04-derived compounds and exceed the values expected from oxygen stoichiometry.Ever since the discovery of high-T, superconductivity in copper-based oxides, there has been intensive activity to further increase T,. 23 Paralleling this search for higher T"numerous theoretical efforts to understand the mechanism of superconductivity in these compounds have been undertaken.From simple charge balance arguments, assuming a 2 valency for oxygen, it was originally thought that Cu(III) be present in both classes of high-T, superconductors, La2-, M"Cu04 (M = Sr, Ba; 2:1:4 compounds) and MBa2Cu307 (M = Y or rare earths; 1:2:3 compounds). Subsequent photoemission results, s 'o however, showed that the proportion of Cu(III) 3d is negligible in these compounds. In order to satisfy charge neutrality, it was therefore proposed that the excess holes required from oxygen stoichiometry are located in the oxygen ligand p band giving rise to 0 species. 5In the present paper we report on results of soft x-ray absorption (XA) studies at the Cu Ltl, tii thresholds in a variety of high-T, superconductors and related compounds. One advantage of XA spectroscopy, in comparison to photoemission, is that the method is more bulk sensitive. Comparing the XA near-edge fine-structure spectra of these compounds with those of CuO and NaCu02, we are able to establish that the holes in the Cu -0 bonds reside primarily on the oxygen ligand sites, and that the amount of Cu(III)-3ds configuration in the ground state is negligible in these superconductors.These findings are essentially in conformity with previous conclusions from photoemission, 5 'o soft XA, 9' and electron-energy-loss spectroscopic studies.The investigated high-T, compounds were prepared by the usual ceramic process starting with the component oxides along with BaCO3, followed by oxygen annealing. All the samples were characterized by powder x-ray diffraction and were found to be single phase. We have also checked their superconducting properties by carrying out standard four-probe resistance measurements. The T, values, for which the resistance dropped below, the level of detection, are given in Table I. For comparison, we have also studied CuO and NaCu02. The preparation of NaCu02 has been described previously.XA studies at TABLE I. Ratio of intensity of peak B to that of peak A in the Cu-L&«XA spectra of the ternary copper-oxide superconductors studied, along with resistively determined transition temperatures T,. hE denotes the separation between peaks B and A in eV. The following error bars are estimated: +'0. 05 for Ie/l~, +' 0.5 K for T"~0...