The Role of the Core Energy in the Vortex Nernst Effect

We present an analytical study of diamagnetism and transport in a film with superconducting phase fluctuations, formulated in terms of vortex dynamics within the Debye-Hückle approximation. We find that the diamagnetic and Nernst signals decay strongly with temperature in a manner which is dictated by the vortex core energy. Furthermore, we find that at high temperatures the ratio between the magnetization and the transverse thermoelectric conductivity tends to a constant, determined by the variation of the core energy near the edges of the system. We use the theory to interpret measurements of underdoped La2-xSrxCuO4 above the critical temperature regime and obtain a considerably better fit to the data than a fit based on Gaussian order-parameter fluctuations. Our results indicate that the core energy in this system scales roughly with the critical temperature. The onset of the Nernst effect lies in a regime which can not be described in terms of a liquid of well defined vortices and involves substantial amplitude fluctuations.