Susceptibility of a mesoscopic superconducting ring

The susceptibility of a single mesoscopic aluminum ring has been studied with an integrated superconducting quantum interference device susceptometer at temperatures near the superconducting critical temperature, and anomalous behavior has been found just above ${\mathrm{T}}_{\mathrm{c}}$ . Below the zero-field critical temperature of 1.266 K we find excellent agreement with a Ginzburg-Landau theory of the susceptibility, and all of the important sample parameters can be accurately determined. The phase-slip transition rates are measured as a function of flux at temperatures down to 0.950 K, and a comparison with the theoretical free-energy barrier heights for these transitions shows that we are able to predict the ratio of the saddle-point energy to the initial-state energy with an accuracy of a few percent. Just above the mean-field ${\mathrm{T}}_{\mathrm{c}}$ we expect the susceptibility to be dominated by thermodynamic fluctuations. A clear signal is found from ${\mathrm{T}}_{\mathrm{c}}$ to 25 mK above ${\mathrm{T}}_{\mathrm{c}}$ , but it is as much as 50 times larger than predicted. The observed phase-slip rates can be used to show that this anomalously large susceptibility just above ${\mathrm{T}}_{\mathrm{c}}$ is not due to temperature fluctuations and that it is not a noise-driven effect.