Observation of the Nernst signal generated by fluctuating Cooper pairs

TL;DR

This paper reports the first clear experimental observation of a Nernst signal caused by fluctuating Cooper pairs above the critical temperature in a superconductor, confirming a long-standing theoretical prediction.

Contribution

It provides the first unambiguous experimental evidence that short-lived Cooper pairs generate a measurable Nernst signal above Tc, validating the theoretical model.

Findings

Nernst signal observed deep into the normal state

Signal magnitude matches theoretical predictions

Confirms Cooper pairs contribute to thermoelectric effects above Tc

Abstract

Long-range order is destroyed in a superconductor warmed above its critical temperature (Tc). However, amplitude fluctuations of the superconducting order parameter survive and lead to a number of well established phenomena such as paraconductivity : an excess of charge conductivity due to the presence of short-lived Cooper pairs in the normal state. According to an untested theory, these pairs generate a transverse thermoelectric (Nernst) signal. In amorphous superconducting films, the lifetime of Cooper pairs exceeds the elastic lifetime of quasi-particles in a wide temperature range above Tc; consequently, the Cooper pairs Nernst signal dominate the response of the normal electrons well above Tc. In two dimensions, the magnitude of the expected signal depends only on universal constants and the superconducting coherence length, so the theory can be unambiguously tested. Here, we report on the observation of a Nernst signal in such a superconductor traced deep into the normal state. Since the amplitude of this signal is in excellent agreement with the theoretical prediction, the result provides the first unambiguous case for a Nernst effect produced by short-lived Cooper pairs.