The Wiedemann-Franz (WF) ratio compares the thermal and electrical conductivities in a metal. We describe a new way to determine its value, based on the thermal Hall conductivity. The technique is applied to copper and to untwinned YBaCuO. In the latter, we uncover a T-linear dependence and suppression of the Hall-channel WF ratio. We discuss the implications of this suppression. The general suppression of the WF ratio in systems with predominant electron-electron scattering is discussed.

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 Tc0. In overdoped La2-xSrxCuO4, we show that the upper critical field Hc2(T)curve does not end at Tc0, but at a much higher temperature. These results imply that Tc0 corresponds to a loss in phase rigidity rather than a vanishing of the pairing amplitude. An intermediate field H*(T)≤Hc2（T） is shown to be the field scale for the flux-flow resistivity.

We present a way to extract the quasiparticle thermal conductivity ke and mean free path in YBa2Cu3Ox using the thermal Hall effect and the field dependence of ke. The results compare well with heat capacity experiments. Moreover, we find a simple relation between the thermal Hall angle θQ and the H dependence of ke, as well as numerical equality between θQ and the electrical Hall angle. The results also reveal an anomalously anisotropic scattering process in the normal state.

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 La22xSrxCuO4 (LSCO) crystals at temperatures 50-100 K above Tc. To pin down the onset temperature Tn of the vortexlike signal in the lightly doped regime (0.03≤x≤0.07), 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 axy and show that its profile provides an objective determination of Tn. With the results, we revise the phase diagram for the fluctuation regime in LSCO to accommodate the lightly doped regime. In the cuprate Bi2Sr22yLayCuO6, 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 Tn relates to the pseudogap temperature T* and the implications of strong fluctuations between the pseudogap state and the d-wave superconducting state.

Two general features of a superconductor, which appear at the critical temperature, are the formation of an energy gap and the expulsion of magnetic flux (the Meissner effect). In underdoped copper oxides, there is strong evidence that an energy gap (the pseudogap1) opens up at a temperature significantly higher than the critical temperature (by 100-220 K). Certain features of the pseudogap suggest that it is closely related to the gap that appears at the critical temperature (for example, the variation of the gap magnitudes around the Fermi surface and their maximum amplitudes are very similar2,3). However, the Meissner effect is absent in the pseudogap state. The nature of the pseudogap state, and its relation (if any) to the superconducting state are central issues in understanding copper oxide superconductivity. Recent evidence suggests that, in the underdoped regime, the Meissner state is destroyed above the critical temperature by strong phase fluctuations1,4-7 (as opposed to a vanishing of the superfluid density). Here we report evidence for vortices (or vortex-like excitations) in La2-xSrxCuO4 at temperatures significantly above the critical temperature. A thermal gradient is applied to the sample in a magnetic field. Vortices are detected by the large transverse electric field produced as they diffuse down the gradient (the Nernst effect). We find that the Nernst signal is anomalously enhanced at temperatures as high as 150 K.