Persistent currents in normal metal rings

We have measured the magnetic response of more than 20 individual mesoscopic gold rings at low temperatures. The rings were characterized one by one using a scanning SQUID microscope, which also enabled in situ measurements of the sensor background. All measured rings show a paramagnetic linear susceptibility and a poorly understood anomaly around zero field, both of which we attribute to unpaired defect spins. The response of some sufficiently small rings also has a component that is periodic in the flux through the ring. Its period is close to $h/e$, and its sign and amplitude vary from ring to ring. Including rings without a detectable periodic response, the amplitude distribution is consistent with predictions for the typical $h/e$ persistent current in diffusive metal rings. The temperature dependence of the response, measured for two rings, is also consistent with theory.