Nernst effect in high-Tcsuperconductors

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 $45\mathrm{T}$ 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.