Systematic Cu L2,3-edge and O K-edge XANES spectroscopy study on the infinite-layer superconductor system, (Sr,La)CuO2

Our systematic O KK-edge and Cu L2,3L2,3-edge XANES study on the infinite-layer superconductor system, (Sr,La) CuO2, reveals both common and a little different features in the electronic structure with/from those reported for the other nn-type copper-oxide superconductor system, (R,Ce)2CuO4. The O KK-edge features show no dependence of the La-substitution level in (Sr,La) CuO2 (in line with the observations made for (R,Ce)2CuO4 upon Ce substitution). This indicates that in nn-type copper-oxide superconductors the doped electrons are not driven to the O site with the resultant Fermi level being located at the bottom of the UHB. At the Cu L3L3-edge, upon electron-doping an additional absorption peak due to nominally monovalent Cu appears about 933.7 eV, and simultaneously the two peaks due to nominally divalent Cu about 931.4 and 936 eV are found to lose spectral weight. Here then the situation differs slightly from that observed for the (R,Ce)2CuO4 system in which the latter peak (due to transitions to the 4s4s orbital) remains unchanged. Based on the spectral features at the Cu L3L3-edge an accurate estimate is made for the average copper valence, V(Cu): a linear decrease of the V(Cu) value with increasing xx is confirmed. Moreover, also revealed is that the highest TcTc value and the largest superconductivity volume fraction correspond to V(Cu)=1.90 in (Sr,La) CuO2 (in agreement with the case of (R,Ce)2CuO4 with the optimum V(Cu) value about 1.88–1.89).