Nernst effect in high-Tcsuperconductors

Abstract

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 $45\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ 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.