Spontaneous fluxoid formation in superconducting loops

We report on the experimental verification of the Zurek-Kibble scenario in an isolated superconducting ring over a wide parameter range. The probability of creating a single flux quantum spontaneously during the fast normal-superconducting phase transition of a wide Nb loop clearly follows a scaling relation on the quenching time tau(Q), as one would expect if the transition took place as fast as causality permits. However, the observed Zurek-Kibble scaling exponent sigma=0.62 +/- 0.15 is two times larger than anticipated for large loops. Assuming Gaussian winding number densities we show that this doubling is well founded for small annuli.