Local view of superconducting fluctuations

Abstract

Superconducting transitions are driven by thermal fluctuations close to the transition temperature, Tc. These fluctuations are averaged out in global measurements, leaving imprints on susceptibility and resistance measurements. Here, we use a scanning superconducting quantum interference device to image thermal superconducting fluctuations in Nb, a conventional BCS superconductor. We observe fluctuations in both space and time which manifest themselves as grains of weaker and stronger diamagnetic response, exhibiting telegraph-like noise as a function of time. Local fluctuations are also found in the imaginary component of the susceptibility demonstrating that the local vortex dissipation can also be used as a probe of the fluctuations. An important outcome of our measurements is the observation and realization that the susceptibility decrease to zero as the temperature is raised towards Tc always occurs in quantized steps irrespective of the sample geometry. The technique described here is expected to be a useful tool for studying the nature of fluctuations in more complex superconductors, providing important information on critical properties such as fluctuation length, time scales, and local viscosity.