Search for spontaneous nucleation of magnetic flux during rapid cooling of YBa₂Cu₃O_7-δ films through T_c

We describe an experimental search for spontaneous formation of flux lines during a rapid quench of thin YBa₂Cu₃O_7-δ films through T_c. This effect is expected according to the Kibble-Zurek mechanism of a creation of topological defects of the order parameter during a symmetry-breaking phase transition. Spontaneously formed vortices were previously observed in superfluid ³He, while a similar experiment in superfluid ⁴He gave negative results. Using a high-T_c superconducting quantum interference device, we measured both the magnetic flux in the sample during a quench with a sensitivity of 20 φ₀/cm², and the field noise which one would expect from flux lines pinned in the film. The sensitivity was sufficient to detect spontaneous flux at a level corresponding to 10^-3 of the prediction. Within our resolution, we saw no evidence for this effect.