Persistent currents and an impressive experiment

A long while ago, I brought up the topic of persistent currents in normal metal rings.  Please click the link to get the context.  The point is, even in a normal metal (as opposed to a superconductor), if you consider a metal ring small enough that the electrons remain quantum mechanically coherent in going about the ring, the electronic wavefunction must remain single-valued.  That means that the quantum mechanical phase accumulated by an electron diffusing around the ring back to its starting point (to speak in a semiclassical way) has to add up to an integer multiple of 2 pi. Since magnetic flux through the ring tweaks the accumulated phase (via the Aharonov-Bohm effect), a persistent current develops in the ring to make sure that the total phase (that from the electron motion and that from the resulting Aharonov-Bohm contribution) add up to a multiple of 2 pi.  As I'd discussed before, these currents and the magnetic fields they produce tend to be quite small and difficult to detect.

To make matters worse, when an electron scatters off static disorder in a solid, it acquires a phase shift that depends on that particular scattering site.  What this really means is, if you consider an ensemble of nominally identical metal rings, you'll actually get some distribution of persistent currents, because each ring has its own particular configuration of disorder.  Now Jack Harris' group at Yale has done a beautiful measurement, looking at many individual rings and examining the statistics of these persistent currents in the ensemble.  They place each ring at the end of a floppy cantilever.  In the presence of a magnetic field, the magnetic dipole moment from the persistent current exerts a torque on the cantilever, and the results can be detected optically via interferometry.  The experiment requires low temperatures, precision fabrication, and very clean technique.  Very nice.