Nernst effect as a probe of superconducting fluctuations in Nb$_{0.15}$Si$_{0.85}$

We present a study of the Nernst effect in thin films of the amorphous superconductor Nb$_{0.15}$Si$_{0.85}$ . A finite Nernst signal was resolved at temperatures well above T$_{c}$ and at relatively high magnetic fields (1). In the zero-field limit and close to T$_{c}$, our results are in very good agreement with a simple relation derived from the theory by Ussishkin, Sondhi and Huse (2) for a two-dimensional superconductor. According to the theory, the magnitude of the Nernst signal generated by the fluctuating Cooper pairs depends only on the superconducting coherence length $\xi$. Far above T$_{c}$ and/or in presence of a finite magnetic field, a departure from this relation is observed. Yet, even in this regime, the amplitude of the Nernst coefficient depends on a single length scale set by $\xi$ and by the magnetic length $l_{B}$. This observation allows to establish a phenomenological relation for the Nernst coefficient, for all magnetic fields and temperatures above T$_{c}$, which depends only on the size of the superconducting fluctuations set by $\xi$ and/or $\l_{B}$.\\ (1) A. Pourret \emph{et al.}, Nature Physics \textbf{2}, 683 - 686 (2006)\\ (2) I. Ussishkin, S. L. Sondhi and D. A. Huse, Phys. Rev. Lett. \textbf{89}, 287001 (2002)