Analysis of the Ghost and Mirror Fields in the Nernst Signal Induced by Superconducting Fluctuations

TL;DR

This paper provides a comprehensive analytical framework for understanding the Nernst signal caused by superconducting fluctuations across various superconductors, fitting experimental data accurately with minimal parameters.

Contribution

It introduces a closed-form analytical expression for the fluctuation-induced Nernst signal applicable to many superconductors, and validates it against experimental data with excellent agreement.

Findings

The analytical expression fits experimental data well using only two parameters.

The model predicts a linear temperature dependence of the ghost critical field.

Experimental data confirms the predicted linearity of the ghost field far above T_c0.

Abstract

We present a complete analysis of the Nernst signal due to superconducting fluctuations in a large variety of superconductors from conventional to unconventional ones. A closed analytical expression of the fluctuation contribution to the Nernst signal is obtained in a large range of temperature and magnetic field. We apply this expression directly to experimental measurements of the Nernst signal in Nb$_x$Si$_{1-x}$ thin films and a URu$_2$Si$_2$ superconductors. Both magnetic field and temperature dependence of the available data are fitted with very good accuracy using only two fitting parameters, the superconducting temperature $T_{\mathrm{c0}}$ and the upper critical field $H_{\mathrm{c2}}$. The obtained values agree very well with experimentally obtained values. We also extract the ghost lines (maximum of the Nernst signal for constant temperature or magnetic field) from the complete expression and also compare it to several experimentally obtained curves. Our approach predicts a linear temperature dependence for the ghost critical field well above $T_{\mathrm{c0}}$. Within the errors of the experimental data, this linearity is indeed observed in many superconductors far from $T_{\mathrm{c0}}$.