Giant Nernst Effect due to Fluctuating Cooper Pairs in Superconductors

A theory of the fluctuation-induced Nernst effect is developed for a two-dimensional superconductor in a perpendicular magnetic field. First, we derive a simple phenomenological formula for the Nernst coefficient, which naturally explains the giant Nernst signal due to fluctuating Cooper pairs. The latter signal is shown to be large even far from the transition and may exceed by orders of magnitude the Fermi liquid terms. We also present a complete microscopic calculation of the Nernst coefficient for arbitrary magnetic fields and temperatures, which is based on the Matsubara-Kubo formalism. It is shown that the magnitude and the behavior of the Nernst signal observed experimentally in disordered superconducting films can be well understood on the basis of superconducting fluctuation theory.

Figure 1

The Aslamazov-Larkin (AL) and density-of-states (DOS) diagrams for the thermoelectric response ${\tilde{\beta }}^{xy}$. The DOS diagram has a symmetric counterpart. The white and black circles correspond to the different heat and electric vertices, the shadowed blocks represent Cooperons, and the wavy lines denote the fluctuation propagator (see text). All objects on these graphs are generally matrices in the Landau basis.

Figure 2

Different asymptotic regions for the fluctuation Nernst effect on the $H\mathrm{\text{−}}T$ phase diagram.

Figure 3

Comparison with experiment. Circles: experimental data for $\underset{H\to 0}{lim}{\beta }^{xy}/H$ vs $ϵ=\ln T/T_c$ obtained for the 12.5-nm-thick ${\mathrm{Nb}}_{0.15}{\mathrm{Si}}_{0.85}$ film [9]. Dashed line: theoretical prediction for the strictly 2D geometry. Solid line: theoretical prediction for the real sample [9].