Will Shanks

One striking phenomenon of mesoscopic physics is the ability of a resistive ring to sustain a constant electrical current, while in thermal equilibrium and in the absence of an external excitation. The observability of persistent currents normal metal rings was first predicted in 1983 [1]. Subsequently, these persistent currents have been studied experimentally several times but with conflicting results due in part to the difficulty of the measurements.

In this work, I present measurements of persistent currents in normal metal rings performed with cantilever torsional magnetometry. With this technique, the typical persistent current (the component that varies randomly from ring to ring) was measured with high sensitivity. I report measured magnitudes ([special characters omitted] 1 pA) over two orders of magnitude smaller than observed in previous studies. These measurements extend the range of temperature and magnetic field over which the typical current has been observed. The wide magnetic field range allowed us to study the effect of magnetic field penetrating the ring. It also enabled the recording of many independent measurements of the current magnitude in a single sample. These independent measurements are necessary to characterize the persistent current magnitude because it is a random quantity. From these measurements of the persistent current, I also characterize the parametric dependence of the typical current on sample orientation and number of rings.

In addition to presenting the experimental results, I thoroughly review the theory of the typical persistent current in the diffusive regime. I begin with the simplest model and build up to the case appropriate for the samples studied in our experiments. I also present in detail the experimental apparatus used to measure the persistent currents.