Observation of the Non-linear Meissner Effect

TL;DR

This paper reports the first clear experimental observation of the non-linear Meissner effect in nodal superconductors, demonstrating how magnetic field influences the penetration depth and distinguishing gap structures.

Contribution

The study provides experimental evidence of the non-linear Meissner effect in specific superconductors and introduces a method to differentiate gap nodes from deep minima.

Findings

Observation of non-linear Meissner effect in CeCoIn$_5$ and LaFePO

Method to distinguish gap nodes from deep gap minima using magnetic field effects

Evidence suggesting KFe$_2$As$_2$ has a non-nodal gap structure

Abstract

A long-standing theoretical prediction is that in clean, nodal unconventional superconductors the magnetic penetration depth $\lambda$, at zero temperature, varies linearly with magnetic field. This non-linear Meissner effect is an equally important manifestation of the nodal state as the well studied linear-in-$T$ dependence of $\lambda$, but has never been convincingly experimentally observed. Here we present measurements of the nodal superconductors CeCoIn$_5$ and LaFePO which clearly show this non-linear Meissner effect. We further show how the effect of a small dc magnetic field on $\lambda(T)$ can be used to distinguish gap nodes from non-nodal deep gap minima. Our measurements of KFe$_2$As$_2$ suggest that this material has such a non-nodal state.