Two series of Y1-xCaxBa2Cu3Oy, /x=0.05 and 0.10, have been studied by O-1s X-ray absorption near-edge spectroscopy using synchrotron radiation. The characteristics of the decrease of Tc with the decrease of the oxygen content, y, are found different for the series of /x=0.05 and /x=0.10. The former reveals the importance of the holes on the apical oxygen sites. Through a cross examination between these two series of samples, direct evidences are observed that the reduction of the number of holes on the apical oxygen sites has a negative effect on the superconductivity in the underdoped region of Ca-doped YBCO system.
Physica C: Superconductivity and its Applications 384(3), pp.314-320