Strongly enhanced diamagnetism and the vortex-Nernst effect in Bi2Sr2CaCu2O8+x

The magnetization of the Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ (Bi-2212) cuprates has been measured using high-resolution cantilever magnetometer. In underdoped Bi-2212, the fluctuation-induced diamagnetism onsets at temperatures much higher than the critical temperature $T_{c0}$. Both the temperature and field dependence of the diamagnetic signals closely match the vortex-Nernst effect measured on the same sample. These new results strongly support the previous finding of the Nernst effect: the vortex-liquid state extends to temperatures well above $T_{c}$ in underdoped cuprates. The transition at $T_{c}$ is caused by the loss of long-range phase coherence among the Cooper pairs. *Supported by NSF (DMR 02-13706).