Multiple fluxoid transitions in mesoscopic superconducting rings

The authors report magnetic measurements of fluxoid transitions in mesoscopic, superconducting aluminum rings. The transitions are induced by applying a flux to the ring so that the induced supercurrent approaches the critical current. In a temperature range near $T_c$, only a single fluxoid enters or leaves at a time, leading to a final state above the ground state. Upon lowering the temperature, several fluxoids enter or leave at once, and the final state approaches the ground state, which can be reached below approximately 0.5 $T_c$. A model based on the widely used time dependent Ginzburg-Landau theory for gapless superconductors can only explain the data if unphysical parameters are used. Heating and quasiparticle diffusion may be important for a quantitative understanding of this experiment, which could provide a model system for studying the nonlinear dynamics of superconductors far from equilibrium.