Anomalous Nernst Effect in the Vortex-Liquid Phase of High-Temperature Superconductors by Layer Decoupling

– Linear diamagnetism is predicted in the vortex-liquid phase of layered superconductors at temperatures just below the mean-field phase transition on the basis of a hightemperature analysis of the corresponding frustrated XY model. The diamagnetic susceptibility, and the Nernst signal by implication, is found to vanish with temperature as (Tc0 − T ) 3 in the vicinity of the meanfield transition at Tc0. Quantitative agreement with recent experimental observations of a diamagnetic signal in the vortex-liquid phase of high-temperature superconductors is obtained. Introduction. – The Abrikosov vortex lattice melts into an extended vortex-liquid phase in high-temperature superconductors subject to an external magnetic field oriented perpendicular to the conducting copper-oxygen planes that make them up[1][2]. The large size in temperature and magnetic field of the vortex-liquid phase can be attributed to such layer anisotropy[3][4][5]. A cross-over from a vortex-line liquid at temperatures just above the melting point of the Abrikosov vortex lattice to a decoupled vortex liquid at higher temperature that shows negligible correlations of the superconducting order parameter across layers is predicted if the vortex lattice in isolated layers melts through a continuous or a weakly first-order phase transition[6]. Such dimensional cross-over is observed experimentally in electronic transport studies of the vortex-liquid phase in moderately anisotropic high-temperature superconductors[7]. The Abrikosov vortex lattice is predicted to sublimate directly into a decoupled vortex liquid at large enough layer anisotropy, on the other hand, if the vortex lattice in isolated layers melts through a first-order phase transition[6]. Electronic transport studies of the mixed phase in extremely layered high-temperature superconductors are consistent with the last sublimation scenario[8]. An anomalous Nernst effect is also observed in the vortex-liquid phase of high-temperature superconductors[9]. In particular, a gradient in temperature along the copper-oxygen planes generates an electric field perpendicular to it along the copper-oxygen planes as well. The low-temperature onset of the anomalous Nernst signal coincides with the melting point of the Abrikosov vortex lattice, while the high-temperature onset can lie above the critical temperature of the superconducting state at zero field. The authors of ref. [9] argue that this