Fluctuation diamagnetism in mesoscopic Aluminum rings

We have measured the magnetic response of mesoscopic, superconducting Al rings using a high sensitivity scanning SQUID microscope. We find that for sufficiently small, quasi one- dimensional rings, the rounding of the superconducting transition due to thermal fluctuations is in good agreement with theoretical preditctions. The main effects are a fluctuation tail in the amplitude of the response near $T_c$ and a suppression of the apparent $T_c$ as determined from the temperature dependence of the superfluid density below the fluctuation tail compared to the extracted mean field $T_c$ by several mK. In larger rings, the tail is less pronounced due to the lack of phase coherence.