Anomalous disappearance of high-Tc superconductivity at high hole concentration in metallic La2-xSrxCuO4

Samples of La_2-xSr_xCuO_4-δ have previously shown a maximum concentration of p=0.15 holes per [CuO₂] unit, because increasing x>0.15 normally induces compensating oxygen vacancies. Annealing samples in 100 bars of oxygen pressure fills the oxygen vacancies and greatly increases the range of accessible hole concentrations, up to p=0.40 (or effectively Cu^+2.40). We find that T_c is constant at ~36 K from p=0.15 to 0.24, where it begins to decrease. Beyond p~=0.32, superconductivity disappears, even though the samples are more conducting.