Fluxoid Conservation by Superconducting Thin Film Rings

Abstract

A torque method for measuring the persistent current in superconducting rings has been used to investigate the conservation of the fluxoid originally predicted by London. The fluxoid through a superconducting ring is composed of two parts, one describing the mechanical angular momentum of the electrons and the other the magnetic flux trapped by the ring. The mechanical angular momentum depends on the penetration depth $\lambda$ and therefore on the temperature. If the fluxoid is conserved, temperature variations should alter the balance between the mechanical and electromagnetic angular momenta. As a consequence, the amount of trapped flux, and hence the persistent current, should vary with temperature even though the ring remains at all times entirely within the pure superconducting state with zero resistance. Very thin films of tin have shown experimentally a decrease in persistent current with increasing temperature and an increase with decreasing temperature which agree with that to be expected on the basis of the fluxoid conservation predicted by London.