Tc Saturation and Possible Electronic Phase Separation in Strongly Overdoped Cuprates

In order to elucidate the unusual superconducting properties of cuprates in the strongly overdoped region, i.e., at hole-doping levels p≳0.4/Cu in the CuO2 plane, we study the structural and superconducting properties of a series of Cu0.75Mo0.25Sr2YCu2O7+x powder samples oxygenated under high pressure using different concentrations of KClO3 up to 35 mol %. The analysis of X-ray diffraction data indicates a high purity ∼90% of all samples and suggests that the concentration, x, of extra oxygen atoms increases with increasing KClO3 concentration. Surprisingly, the Tc values remain nearly constant within the 80–85 K range independent of KClO3 concentration, which suggests a scenario of Tc saturation. In order to account for this unexpected behaviour, we put forward the hypothesis that overdoping enhances the density of unpaired holes, which is supported by the observation of large values of the Sommerfeld coefficient in all samples. We therefore propose a scenario of electronic phase separation between normal and superconducting holes.