Study of the onset of superconductivity in underdoped La2-xSrxCuO4

The origin of the appearance of superconductivity in the high-transition-temperature (high-T-c) copper oxides has remained a subject of active inquiry. In order to address this issue, we chose La2-xSrxCuO4 as a model system and succeeded in fine controlling the hole concentration around the composition where T-c appears. T-c did not emerge smoothly at a critical hole concentration, x(c). T-c and the absolute value of the Meissner signal below T-c increased progressively with Sr content x > x(c). Measurements of magnetization versus temperature under hydrostatic pressure (P) are also reported for La2-xSrxCuO4, which has no charge reservoir. A dT(c)/dP > 0 found in 0.05 < x < 0.22 peaks out at x approximate to 0.07. Comparison between these results and predictions from existing models for the high-T-c superconductivity has been made.