Comparative XANES study on the two electron-doped high-Tc superconductor systems, (Sr,La)CuO2 and (Nd,Ce)2CuO4

Here we employ high-quality samples of (Sr1−xLax)CuO2 and (Nd2−xCex)CuO4 and XANES spectroscopy at O-K, Cu-L2,3 and Ce-M4,5 edges to gain comprehensive understanding of the electronic structure and doping in n-type high-Tc superconductors. Not only common but also slightly different features are revealed for the two systems. From O-K-edge spectra, the UHB is found essentially independent of the electron-doping level for both the systems, in line with our understanding that the doped electrons do not go to the O site in n-type copper-oxide superconductors. Another common observation is that the main CuII peak at the Cu-L3 edge (due to transitions to the CuII-3d orbitals) systematically decreases in intensity upon electron doping, hence verifying the fact that the doped electrons go to the Cu site. The difference then between the two systems is that in (Sr1−xLax)CuO2 the weaker CuII peak due to transitions to the CuII-4s orbital depends on the degree of doping. Moreover, it was found that with increasing x, electron density increases much faster in (Sr1−xLax)CuO2 than in (Nd2−xCex)CuO4. This is a consequence of two phenomena: a tiny increase in oxygen content concomitant to the CeIV-for-NdIII substitution and the somewhat lower Ce-valence value of +3.8 compared to the nominal tetravalent state.