Diamagnetism and Cooper pairing above T_c in cuprates

In the cuprate superconductors, Nernst and torque magnetization experiments have provided evidence that the disappearance of the Meissner effect at T_c is caused by the loss of long-range phase coherence, rather than the vanishing of the pair condensate. Here we report a series of torque magnetization measurements on single crystals of La_2−xSr_xCuO₄ (LSCO), Bi₂Sr_2−yLa_yCuO₆ (Bi 2201), Bi₂Sr₂CaCu₂O₈₊, and optimal YBa₂Cu₃O₇. Some of the measurements were taken to fields as high as 45 T. Focusing on the magnetization above T_c, we show that the diamagnetic term M_d appears at an onset temperature T high above T_c. We construct the phase diagram of both LSCO and Bi 2201 and show that T agrees with the onset temperature of the vortex Nernst signal T. Our results provide thermodynamic evidence against a recent proposal that the high-temperature Nernst signal in LSCO arises from a quasiparticle contribution in a charge-ordered state. ©2010 The American Physical Society