Nonlinear Meissner Effect in Nb3Sn Coplanar Resonators

We investigated the nonlinear Meissner effect (NLME) in Nb3Sn 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 Nb3Sn films, which can mimic the NLME expected in a clean d-wave superconductor.