|Ph.D Student||Golubchik Daniel|
|Subject||Magneto-Optical Measurements of Spontaneous Creation|
of Flux during a Rapid Phase Transition in
|Department||Department of Physics||Supervisor||Professor Emeritus Emil Polturak|
|Full Thesis text|
According to a prediction by Kibble and Zurek, topological defects of the order parameter are spontaneously created during nonequilibrium phase transitions. This prediction (the Kibble-Zurek model) is relevant in phase transitions both in cosmological and solid state systems. In superconductors these topological defects are quantized vortices. Several conflicting predictions exist with respect to the density of vortices, the correlations between them, and the importance of gauge fields.
In order to test this model we developed a novel magneto-optical system with a single vortex resolution. The sample (thin film of niobium) was quench-cooled through the phase transition at rates up to 109 K/s. We found that vortices are spontaneously created in the sample even at zero external field. Density and spatial distribution of spontaneously created vortices were measured. Our results are in agreement with the Kibble-Zurek scenario of spontaneous vortex creation. The measured vortex-vortex correlation function is always negative and decays exponentially on a scale of the freeze-out correlation length. The correlations are insensitive to the presence of a gauge field.
Additional experiment performed using the same experimental system and samples is described in Appendix A in the thesis. In this experiment we measure the dynamics of individual vortices in a superconducting film. The motion took place while the film was heated close to Tc, where pinning and viscous forces are relatively small. Under these conditions, the dominant force is the magnetic repulsion between vortices. The extent of the motion of individual vortices is consistent with a vortex mass of few electronic masses per lattice constant at 0.97 Tc.