High Field Phase Diagram of Cuprates Derived from the Nernst Effect

Measurements of the Nernst signal in the vortex-liquid state of the cuprates to high fields (33 T) reveal that vorticity extends to very high fields even close to the zero-field critical temperature T_c0. In overdoped La_2-xSr_xCuO₄, we show that the upper critical field H_c2(T) curve does not end at T_c0, but at a much higher temperature. These results imply that T_c0 corresponds to a loss in phase rigidity rather than a vanishing of the pairing amplitude. An intermediate field H^*(T)<<H_c2(T) is shown to be the field scale for the flux-flow resistivity.