Observation of magnetic flux generated spontaneously during a rapid quench of superconducting films

We report observations of spontaneous formation of magnetic flux lines during a rapid quench of YBa2Cu3O7-delta films through T-c. This effect is predicted according to the Kibble-Zurek mechanism of creation of topological defects of the order parameter during a symmetry-breaking phase transition. Our previous experiment, at a quench rate of 20 K/s, gave null results. In the present experiment, the quench rate was increased to >10(8) K/s. The amount of spontaneous flux increases weakly with the cooling rate.