|Ph.D Student||Maniv Ariel|
|Subject||Measurements of Spontaneous Magnetic Flux Generated during|
a Rapid Thermal Quench of a Superconductor
|Department||Department of Physics||Supervisor||Professor Emeritus Emil Polturak|
We report observations of spontaneous formation of magnetic flux lines during a rapid quench of YBCO7-d high temperature superconducting films through the transition temperature. 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. In the experiment we use a high Tc SQUID in order to measure changes in net flux during the quench as a function of cooling rate. Our previous experiment, at a quench rate of 20K/sec, gave null results. In the present experiment, the quench rate was increased to above 108 K/sec. The amount of spontaneous flux increases weakly with the cooling rate, similar to the prediction based on the Kibble-Zurek scenario, hence supporting it. In addition, results of non homogeneous experiments, in which the samples where illuminated in the form of a “hot-stripe”, are reported. Formation of spontaneous flux was also observed in these experiments, but with a few dissimilarities. In addition to the fast (less than 10ms) signal, a much larger and slower signal followed, developing on a time scale ranging between 1- 10ms. In the "hot-stripe" experiments the signal decreased with cooling rate. In these experiments the signal also increased with pulse energy. Since the temperature gradient across the film increases with pulse energy (hence the signal increases with the temperature gradient) , and the typical decay time of thermal gradients in the film is similar to the time scale it takes the "hot-stripe" signal to develop (~10ms) we speculate that the non-homogeneous signal originates from a thermoelectric effect.