Onset of the vortexlike Nernst signal above (formula presented) in (formula presented) and (formula presented)

The diffusion of vortices down a thermal gradient produces a Josephson signal which is detected as a vortex Nernst effect. In a recent report by Xu et al. [Nature 406, 486 (2000)], an enhanced Nernst signal identified with vortex-like excitations was observed in a series of (formula presented) (LSCO) crystals at temperatures 50–100 K above (formula presented) To pin down the onset temperature (formula presented) of the vortexlike signal in the lightly doped regime (formula presented) we have reanalyzed the carrier contribution to the Nernst signal in detail. By supplementing Nernst measurements with thermopower and Hall-angle data, we isolate the off-diagonal Peltier conductivity (formula presented) and show that its profile provides an objective determination of (formula presented) With the results, we revise the phase diagram for the fluctuation regime in LSCO to accommodate the lightly doped regime. In the cuprate (formula presented) we find that the carrier contribution is virtually negligible for y in the range 0.4–0.6. The evidence of an extended temperature interval with vortexlike excitations is even stronger in this system. Finally, we discuss how (formula presented) relates to the pseudogap temperature (formula presented) and the implications of strong fluctuations between the pseudogap state and the d-wave superconducting state.