Electron energy-loss spectroscopy of Sr2CuO2+δCl2−y: Overdoping and role of apical oxygen for high-Tc superconductivity

By measuring O 1s and Cu 2p3/2 absorption edges of Sr2CuO2+δCl2−y using orientation-dependent high-energy electron energy-loss spectroscopy, the changes of the unoccupied electronic structure close to the Fermi level induced by stoichiometry variations have been explored. An interesting hole redistribution between the in-plane O 2p orbitals and apical O 2p orbitals induced by the doping level has been revealed in this system. For the low-doped samples in which superconductivity is absent, the holes reside predominantly on the oxygen sites of the CuO2 plane, while for the high-doped samples in which superconductivity appears, considerable holes are found to enter the apical oxygen sites. This result reveals clearly that the apical O 2p holes play an important role in the occurrence of superconductivity in the ‘apical oxygen doped’ Sr2CuO2+δCl2−y system.