Diamagnetism and Cooper pairing above<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mi>T</mml:mi><mml:mi>c</mml:mi></mml:msub></mml:mrow></mml:math>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 ${\text{La}}_{2\ensuremath{-}x}{\text{Sr}}_{x}{\text{CuO}}_{4}$ (LSCO), ${\text{Bi}}_{2}{\text{Sr}}_{2\ensuremath{-}y}{\text{La}}_{y}{\text{CuO}}_{6}$ (Bi 2201), ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{CaCu}}_{2}{\text{O}}_{8+\ensuremath{\delta}}$, and optimal ${\text{YBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{7}$. 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 ${T}_{onset}^{M}$ high above ${T}_{c}$. 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 ${T}_{onset}^{\ensuremath{\nu}}$. 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.

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