Diamagnetism and Cooper pairing above $T_c$ in cuprates

Abstract

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 $\mathrm{L}{\mathrm{a}}_{2-\mathrm{x}}\mathrm{S}{\mathrm{r}}_{\mathrm{x}}\mathrm{Cu}{\mathrm{O}}_4$ (LSCO), $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{r}}_{2-\mathrm{y}}\mathrm{L}{\mathrm{a}}_{\mathrm{y}}\mathrm{Cu}{\mathrm{O}}_6$ (Bi 2201), $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{r}}_2\mathrm{CaC}{\mathrm{u}}_2{\mathrm{O}}_{8+\hat{\mathrm{I}}´}$ (Bi 2212) and optimal $\mathrm{YB}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_7$. 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 $T^{\hat{I}}{½}_{onset}$. 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.

10 pages, 11 figures. Final version with revised text and expanded discussion and 1 new figure (Fig. 10) and 1 modified fig (Fig. 11). Some new references