Fluctuation superconductivity in mesoscopic aluminum rings

Fluctuations are important near phase transitions, where they can be difficult to describe quantitatively. Superconductivity in mesoscopic rings is particularly intriguing because the critical temperature is an oscillatory function of magnetic field. There is an exact theory for thermal fluctuations in one-dimensional superconducting rings, which are therefore expected to be an excellent model system. We measured the susceptibility of many rings, one ring at a time, by using a scanning superconducting quantum interference device that can isolate magnetic signals that are seven orders of magnitude smaller than applied flux. We find that the fluctuation theory describes the results and that a single parameter characterizes the ways in which the fluctuations are especially important at magnetic fields where the critical temperature is suppressed.