Zurek–Kibble Symmetry Breaking Process in Superconducting Rings; Spontaneous Fluxon Formation in Annular Josephson Tunnel Junctions

摘要

We report on new investigations of spontaneous symmetry breaking in non-adiabatic phase transitions. This Zurek–Kibble (ZK) process is mimicked in solid state systems by trapping of magnetic flux quanta, fluxons, in a long annular Josephson tunnel junction quenched through the normal-superconducting transition. A trapped fluxon unambiguously is detected as a zero-field step in the DC I-V characteristic. Experimentally we plot the fluxon trapping probability versus the quench rate, varied over 4 decades. An allometric scaling behavior is found. By fitting to the theoretical curve we get $sigmasimeq0.5$ for the ZK critical scaling exponent $sigma$, which does not agree with an earlier theoretical prediction of $sigma=0.25$ . A novel theory based on the proximity effect leading to $sigma=0.50$ has been proposed. The dependence of the gap voltage on temperature is measured and used for precise monitoring of the fast temperature variation during the quench.