Same Superconducting Criticality for Underdoped and Overdoped La_2-xSr_xCuO₄ Single Crystals

By measuring the superconducting diamagnetic moments for an underdoped and an overdoped La_2-xSr_xCuO₄ single crystal with equal quality and roughly equal transition temperatures, it is found that the underdoped sample has only one transition which corresponds to H_c2, but the overdoped sample has two transitions with the higher one at H_c2. Further investigation reveals the same upper-critical field H_c2 for both samples although the overall charge densities are very different, indicating the possibility of a very direct and detailed equivalence of the superconducting condensation process in the two doping limits. The second transition for the overdoped sample can be understood as the bulk coupling between the superconducting clusters produced by macroscopic phase separation.