Hole states in (Cd0.5Pb0.5)Sr2(CaxY1 – x)Cu2O7 studied by X-ray absorption spectroscopy

Abstract

The hole distribution of underdoped and optimum doped states in (Cd_0.5Pb_0.5)Sr₂(Ca_xY_1 – x)Cu₂O₇ compounds has been investigated by high-resolution oxygen K-edge and copper L-edge X-ray absorption near-edge-structure spectra. Near the oxygen 1s edge, a pre-edge peak with a maximum at ≈528.3 eV was found, which is ascribed to the excitations of oxygen 1s electrons to 2p hole states located in the CuO₂ planes. The intensity of this pre-edge peak increased with calcium doping for 0 ⩽ x ⩽ 0.5. This demonstrates that the chemical substitution of Ca²⁺ for Y³⁺ in (Cd_0.5Pb_0.5)Sr₂(Ca_xY_1 – x)Cu₂O₇ gives rise to an increase in hole concentration within the CuO₂ planes and a change from a semiconductor to a superconductor with T_c up to 30 K. The results of the copper L-edge absorption are consistent with those observed for oxygen 1s X-ray absorption.