Chemical Control of Underdoped and Overdoped States in Y(Ba2 − ySry)Cu3O6 + δ

The chemical control of underdoped and overdoped states in the Y(Ba2 − ySry)Cu3O6 + δ (δ∼0.1 and 0.9) compounds has been observed by high-resolution O K-edge X-ray-absorption near-edge-structure spectra. The chemical substitution of Sr for Ba in the fully-oxygenated Y(Ba2−ySry)Cu3O6 + δ (δ∼0.9) compounds gives rise to high hole concentrations within both the CuO2 planes and the out-of-plane sites, leading to the overdoped state and the decrease in the superconducting transition temperature from 92 K for y=0 to 84 K for y=0.8. In contrast, an increase in the Sr content in the oxygen-deficient Y(Ba2 − ySry)Cu3O6 + δ (δ∼0.1) compounds did not indicate superconductivity. The oxygen-deficient compounds exhibit the underdoped state due to the low hole concentration.