By measuring the superconducting diamagnetic moments for an underdoped and an overdoped ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ single crystal with equal quality and roughly equal transition temperatures, it is found that the underdoped sample has only one transition which corresponds to ${\mathrm{H}}_{c2\mathrm{}}$, but the overdoped sample has two transitions with the higher one at ${\mathrm{H}}_{c2\mathrm{}}$. Further investigation reveals the same upper-critical field ${\mathrm{H}}_{c2\mathrm{}}$ 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.

• Received 21 March 2000

DOI:https://doi.org/10.1103/PhysRevLett.85.2805