Credit: © 2009 APS

The phenomenon of electrical currents flowing forever — persistent currents — in ring-shaped superconducting structures exposed to external magnetic fields is well understood in terms of the Meissner effect. The prediction that persistent currents could exist in small, normal metal rings was confirmed 20 years ago, but the lack of tools for analysing individual rings has led to significant differences between theoretical predictions and experimental observations.

Kathryn Moler and colleagues at Stanford University and the University of Colorado Denver have now used a scanning SQUID (superconducting quantum interference device) microscope for the precise measurement of persistent currents in individual, cooled, micrometre-size gold rings, induced by an external magnetic field1. Their results agree with theories predicting the distribution of the magnitudes of persistent currents to be h/e, where h is Planck's constant and e the electron charge.

The 1–2-μm-diameter, 140-nm-thick gold rings were fabricated by electron beam lithography and the highly compact micro-SQUID sensor — composed of a 13-μm-coil for generating and modulating the magnetic field, a 4.6-μm-sensing-coil, and a noise cancelling coil — was scanned laterally over a sample with 33 gold rings.

These results will enhance the understanding of persistent currents in micro-metal rings, as well as superconductor structures.