  In superconductors, and in other systems with a local U(1) gauge invariance, there are two mechanisms that form topological defects in phase transitions. In addition to the standard Kibble mechanism, thermal fluctuations of the magnetic field also lead to defect formation. This mechanism is specific to local gauge theories, predicts a qualitatively different spatial defect distribution and is the dominant source for topological defects in slow transitions. I review the arguments that lead to these conclusions and discuss possibilities of testing the scenario in superconductor experiments. 
Local gauge invariance and formation of topological defects