Diamagnetism and Cooper pairing above $T_c$ in cuprates

TL;DR

This study presents thermodynamic evidence of diamagnetism and Cooper pairing persisting above the critical temperature in cuprate superconductors, challenging the idea that the Nernst signal is solely due to quasiparticles.

Contribution

The paper provides direct magnetization measurements showing diamagnetism above $T_c$ and correlates it with vortex Nernst signals, offering new insights into the pseudogap phase.

Findings

Diamagnetism appears at a temperature well above $T_c$.

The onset temperature of diamagnetism matches the vortex Nernst signal onset.

Results challenge the quasiparticle explanation for the high-temperature Nernst signal.

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 $\mathrm{La_{2-x}Sr_xCuO_4}$ (LSCO), $\mathrm{Bi_2Sr_{2-y}La_yCuO_6}$ (Bi 2201), $\mathrm{Bi_2Sr_2CaCu_2O_{8+\delta}}$ (Bi 2212) and optimal $\mathrm{YBa_2Cu_3O_7}$. 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 $T^M_{onset}$ high above $T_c$. We construct the phase diagram of both LSCO and Bi 2201 and show that $T^M_{onset}$ agrees with the onset temperature of the vortex Nernst signal $T^{\nu}_{onset}$. 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.