We have observed spontaneous fluxoid generation in thin film rings of the amorphous superconductor ${\mathrm{M}\mathrm{o}}_3\mathrm{S}\mathrm{i}$, cooled through the normal-superconducting transition, as a function of quench rate and externally applied magnetic field, using a variable sample temperature scanning SQUID microscope. Our results can be explained using a model of freeze-out of thermally activated fluxoids, mediated by the transport of bulk vortices across the ring walls. This mechanism is complementary to a mechanism proposed by Kibble and Zurek, which only relies on causality to produce a freeze-out of order parameter fluctuations.

• Received 19 February 2003

DOI:https://doi.org/10.1103/PhysRevLett.90.257001