Onset of the vortexlike Nernst signal above Tc in La2-xSrxCuO4 and Bi2Sr2-yLaCuO6 -: art. no. 224519

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 at, [Nature 406, 486 (2000)], an enhanced Nernst signal identified with vortex-like excitations was observed in a series of La2-xSrxCuO4 (LSCO) crystals at temperatures 50-100 K above T-c. To pin down the onset temperature T-v of the vortexlike signal in the lightly doped regime (0.03 less than or equal tox less than or equal to0.07), 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 alpha (xy) and show that its profile provides an objective determination of T-v. With the results. we revise the phase diagram for the fluctuation regime in LSCO to accommodate the lightly doped regime. In the cuprate Bi2Sr2-yLayCuO6, 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 T-v relates to the pseudogap temperature T* and the implications of strong fluctuations between the pseudogap state and the d-wave superconducting state.