Magnetic field dependence of the critical current in stacked Josephson junctions. Evidence for fluxon modes in Bi 2Sr 2CaCu 2O 8+ x mesas

Abstract

Modulation of the critical current across layers, I c( H), of stacked Josephson junctions (SJJs) as a function of an applied magnetic field parallel to the junction planes is studied theoretically and experimentally for different junction lengths and coupling parameters. It is shown that the I c( H) patterns of long SJJs are very complicated without periodicity in H. This is due to interaction between junctions in the stack. This, in turn, gives rise to the existence of multiple quasi-equilibrium Josephson fluxon modes and submodes which are different with respect to the symmetry of the phase and the fluxon sequence in SJJs. The critical current of long SJJs is multiple valued and is governed by switching between energetically close fluxon modes/submodes. Due to this, the probability distribution of the critical current may become wide and may consist of multiple maxima each representing a particular mode/submode. Experimentally, multiple branched I c( H) patterns and multiple maxima in the I c probability distribution were observed for Bi 2Sr 2CaCu 2O 8+ x intrinsic SJJs, which are in a good agreement with numerical simulations and support the idea of having different quasi-equilibrium fluxon modes/submodes in intrinsic SJJs.