Systematic Cu L2,3-edge and O K-edge XANES spectroscopy study on the infinite-layer superconductor system, (Sr,La)CuO 2

Abstract

Our systematic O K -edge and Cu L 2 , 3 -edge XANES study on the infinite-layer superconductor system, (Sr,La) CuO 2, reveals both common and a little different features in the electronic structure with/from those reported for the other n -type copper-oxide superconductor system, ( R , Ce ) 2 CuO 4 . The O K -edge features show no dependence of the La-substitution level in (Sr,La) CuO 2 (in line with the observations made for ( R , Ce ) 2 CuO 4 upon Ce substitution). This indicates that in n -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 L 3 -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 ) 2 CuO 4 system in which the latter peak (due to transitions to the 4 s orbital) remains unchanged. Based on the spectral features at the Cu L 3 -edge an accurate estimate is made for the average copper valence, V (Cu) : a linear decrease of the V (Cu) value with increasing x is confirmed. Moreover, also revealed is that the highest T c value and the largest superconductivity volume fraction correspond to V ( Cu ) = 1.90 in (Sr,La) CuO 2 (in agreement with the case of ( R , Ce ) 2 CuO 4 with the optimum V (Cu) value about 1.88–1.89).