Diamagnetic Orbital Response of Mesoscopic Silver Rings

We report measurements of the flux-dependent orbital magnetic susceptibility of an ensemble of ${10}^5$ disconnected silver rings at 217 MHz. Because of the strong spin-orbit scattering rate in silver this experiment is a test of existing theories on ensemble averaged persistent currents. Below 100 mK the rings exhibit a magnetic signal with a flux periodicity of $h/2e$ consistent with averaged persistent currents, whose amplitude is of the order of 0.3 nA. The sign of the oscillations indicates unambiguously diamagnetism in the vicinity of zero magnetic field. This sign is a priori not consistent with theoretical predictions for average persistent currents. We discuss several possible explanations of this result.