Nernst effect as a probe of superconducting fluctuations in disordered thin films

TL;DR

This study demonstrates that the Nernst effect effectively probes superconducting fluctuations in disordered thin films, revealing detailed insights into fluctuation regimes and the nature of the superconducting state.

Contribution

It provides experimental evidence that the Nernst coefficient directly reflects superconducting fluctuations and their evolution across different regimes in disordered thin films.

Findings

Nernst coefficient is dominated by superconducting fluctuations in studied films.

Quantitative agreement with Gaussian fluctuation theory near critical temperature.

Different temperature evolutions of Nernst signal distinguish vortex states and phase fluctuations.

Abstract

In amorphous superconducting thin films of $Nb_{0.15}Si_{0.85}$ and $InO_x$, a finite Nernst coefficient can be detected in a wide range of temperature and magnetic field. Due to the negligible contribution of normal quasi-particles, superconducting fluctuations easily dominate the Nernst response in the entire range of study. In the vicinity of the critical temperature and in the zero-field limit, the magnitude of the signal is in quantitative agreement with what is theoretically expected for the Gaussian fluctuations of the superconducting order parameter. Even at higher temperatures and finite magnetic field, the Nernst coefficient is set by the size of superconducting fluctuations. The Nernst coefficient emerges as a direct probe of the ghost critical field, the normal-state mirror of the upper critical field. Moreover, upon leaving the normal state with fluctuating Cooper pairs, we show that the temperature evolution of the Nernst coefficient is different whether the system enters a vortex solid, a vortex liquid or a phase-fluctuating superconducting regime.