Nonlinear Meissner effect in Nb3Sn coplanar resonators

TL;DR

This study measures the nonlinear Meissner effect in Nb3Sn coplanar resonators, revealing a nearly linear increase in penetration depth with magnetic field due to grain boundary effects, challenging conventional quadratic expectations.

Contribution

It demonstrates that grain boundaries in Nb3Sn films can mimic the nonlinear Meissner effect typically associated with d-wave superconductors, under equilibrium conditions.

Findings

Observed nearly linear increase of penetration depth with magnetic field.

Identified grain boundaries as the cause of NLME-like behavior.

Used low rf power probing to measure effects under equilibrium conditions.

Abstract

We investigated the nonlinear Meissner effect (NLME) in Nb$_3$Sn thin film coplanar resonators by measuring the resonance frequency as a function of a parallel magnetic field at different temperatures. We used low rf power probing in films thinner than the London penetration depth $\lambda(B)$ to significantly increase the field onset of vortex penetration and measure the NLME under equilibrium conditions. Contrary to the conventional quadratic increase of $\lambda(B)$ with $B$ expected in s-wave superconductors, we observed a nearly linear increase of the penetration depth with $B$. We concluded that this behavior of $\lambda(B)$ is due to weak linked grain boundaries in our polycrystalline Nb$_3$Sn films, which can mimic the NLME expected in a clean d-wave superconductor.