MAGNETIC-FLUX TRANSITIONS IN SUPERCONDUCTING LOOPS CLOSED WITH A LOW-CAPACITANCE JOSEPHSON JUNCTION DUE TO THERMAL-ACTIVATION

Superconducting rings that are interrupted by a weak superconducting junction can attain two different metastable flux states when they are placed in a magnetic field of appropriate strength. In this article we present the measurements of the mean lifetime of the flux states in the double well potential as a function of the temperature for two types of weak link devices, the evaporated planar ring and the point-contact device. It appears that the transition probability is too small compared to the classical activation theory of Kramers and its modern quantum versions. Two possible explanations of this discrepancy are discussed: (1) the adiabatic instead of isothermal behaviour during the stochastical flux transitions and (2) deviations from the sinusoidal supercurrent-phase relation. Another result obtained from our measurements is the deviation of the mean lifetimes over the flux states as a function of the applied magnetic flux from the quasi-Boltzmann distribution. It appears that this problem can be solved experimentally by using specially designed microwave filters.