Abstract
The observation of a large Nernst signal in an extended region above the critical temperature in hole-doped cuprates provides evidence that vortex excitations survive above . 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 which provide strong evidence for this phase-disordering scenario. Measurements of in fields up to 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 , underscores the continuity between the vortex-liquid state below and the Nernst region above . The upper critical field (depairing field) determined by the hill profile (in slightly UD to overdoped samples) displays an anomalously weak dependence, which is consistent with the phase-disordering scenario. We contrast the Nernst results and behavior in hole-doped and electron-doped cuprates. Contour plots of in the - plane clearly bring out the continuous extension of the low- vortex liquid state into the high- Nernst region in hole-doped cuprates (but not in the electron-doped cuprate). The existence of an enhanced diamagnetic magnetization that survives to intense above is obtained from torque magnetometry. The observed scales accurately like above , confirming that the large Nernst signal is associated with local diamagnetic supercurrents that persist above . We emphasize implications of the new features in the phase diagram implied by the high-field results and discuss relevant theories.
21 More- Received 23 September 2005
DOI:https://doi.org/10.1103/PhysRevB.73.024510
©2006 American Physical Society