MAGNETIC-FIELD DEPENDENCE OF THERMAL EXCITATIONS IN JOSEPHSON-JUNCTIONS

We have measured the rate of escape out of the zero-voltage state in J osephson tunnel junctions as a function of the applied magnetic field. A marked difference is found in the behavior of long and small juncti ons. In all cases, the statistical distribution of the switching curre nts can be described using a Kramers model for the escape process, whe re the barrier to be overcome is the Josephson barrier and the activat ion energy is due to an effective temperature T-e. For small junctions T-e coincides, as expected, with the thermodynamic temperature, regar dless of the applied magnetic field. For long junctions instead it is found that the escape temperature depends markedly on the magnetic fie ld and on the junction geometry (inline or overlap), suggesting a clos e relationship with the magnetic field distribution inside the junctio n.