Dynamical evidence of critical fields in Josephson junctions

We study the dynamical stability of phase configurations generated by an external magnetic field in long Josephson junctions. Depending on the value of the field, the penetration of the vortex lines through the boundary of the junctions gives rise to different dynamical regimes whose nature is characterized by measurements of Fiske singularities in the current-voltage characteristics of the junctions. The magnetic-field dependence of the height of these singularities is compared with numerical simulations of the sine-Gordon equation and low-temperature scanning electron microscopy of the junctions is performed in order to validate the dynamical patterns. For all the junctions that we have investigated, given their maximum pair current density j{sub c} and the Josephson penetration depth {lambda}{sub j}, we find that the external magnetic field that equals the critical value H{sub 0}=2{lambda}{sub j}j{sub c}, responsible for the trapping of a single static flux-quantum in the junction, plays a dominant role in establishing dynamical phase configurations. Both simulations and low-temperature scanning electron microscopy show complete analogy between the dynamical patterns of long and small-area junctions when H{sub 0} is exceeded. {copyright} {ital 1997} {ital The American Physical Society}