Measuring the superconducting coherence length in thin films using a two-coil experiment

We present measurements of the superconducting coherence length xi in thin (d <= 100-angstrom) films of MoGe and Nb using a combination of linear and nonlinear mutual inductance techniques. As the alternating current in the drive coil is increased at a 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 (h) over bar/xi and the unbinding barrier vanishes. We compare measurements of xi performed 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.