We present measurements of the superconducting coherence length \xi\ in thin (d < 100 \AA) films of MoGe alloy and Nb using a combination of linear and nonlinear mutual inductance techniques. As the alternating current in the drive coil is increased at fixed temperature, we see a crossover from linear to nonlinear coupling to the pickup coil, consistent with the unbinding of vortex-antivortex pairs as the peak pair momentum nears \hbar\/\xi\ and the unbinding barrier vanishes. We compare measurements of \xi\ made by this mutual inductance technique to values determined from the films' upper critical fields, thereby confirming the applicability of a recent calculation of the upper limit on a vortex-free state in our experiment.
@article{arxiv.1403.6856,
title = {Measuring the superconducting coherence length in thin films using a two-coil experiment},
author = {John Draskovic and Thomas R. Lemberger and Brian Peters and Fengyuan Yang and Jaseung Ku and Alexey Bezryadin and Song Wang},
journal= {arXiv preprint arXiv:1403.6856},
year = {2014}
}