Diamagnetism and Cooper pairing above Tc in cuprates

In the cuprate superconductors, Nernst and torque magnetization experiments have provided evidence that the disappearance of the Meissner effect at Tc 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 La2-x Srx CuO4 (LSCO), Bi2 Sr2-y Lay CuO6 (Bi 2201), Bi2 Sr2 CaCu2 O8+δ, and optimal YBa2 Cu3 O7. Some of the measurements were taken to fields as high as 45 T. Focusing on the magnetization above Tc, we show that the diamagnetic term Md appears at an onset temperature T onset M high above Tc. We construct the phase diagram of both LSCO and Bi 2201 and show that T onset M agrees with the onset temperature of the vortex Nernst signal T onset ν. 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.