Nernst effect in high-T-c superconductors

The observation of a large Nernst signal e(N) in an extended region above the critical temperature T-c in hole-doped cuprates provides evidence that vortex excitations survive above T-c. The results support the scenario that superfluidity vanishes because long-range phase coherence is destroyed by thermally created vortices (in zero field) and that the pair condensate extends high into the pseudogap state in the underdoped (UD) regime. We present a series of measurements to high fields H which provide strong evidence for this phase-disordering scenario. Measurements of e(N) in fields H up to 45 T reveal that the vortex Nernst signal has a characteristic tilted-hill profile, which is qualitatively distinct from that of quasiparticles. The hill profile, which is observed above and below T-c, underscores the continuity between the vortex-liquid state below T-c and the Nernst region above T-c. The upper critical field (depairing field) H-c2 determined by the hill profile (in slightly UD to overdoped samples) displays an anomalously weak T dependence, which is consistent with the phase-disordering scenario. We contrast the Nernst results and H-c2 behavior in hole-doped and electron-doped cuprates. Contour plots of e(N)(T,H) in the T-H plane clearly bring out the continuous extension of the low-T vortex liquid state into the high-T Nernst region in hole-doped cuprates (but not in the electron-doped cuprate). The existence of an enhanced diamagnetic magnetization M that survives to intense H above T-c is obtained from torque magnetometry. The observed M scales accurately like e(N) above T-c, confirming that the large Nernst signal is associated with local diamagnetic supercurrents that persist above T-c. We emphasize implications of the new features in the phase diagram implied by the high-field results and discuss relevant theories.