Magnetic field dependence of the critical current in stacked Josephson junctions. Evidence for fluxon modes in Bi2Sr2CaCu2O8+x mesas

TL;DR

This study investigates how magnetic fields influence the critical current in stacked Josephson junctions, revealing complex fluxon modes and multiple critical current states, supported by both theoretical analysis and experimental data on Bi2Sr2CaCu2O8+x mesas.

Contribution

It provides new insights into fluxon mode interactions in long SJJs, combining theoretical modeling with experimental evidence to explain complex Ic(H) patterns.

Findings

Long SJJs exhibit non-periodic, complex Ic(H) patterns due to fluxon interactions.

Multiple quasi-equilibrium fluxon modes cause multiple maxima in critical current distributions.

Experimental data on Bi2Sr2CaCu2O8+x mesas confirm theoretical predictions of fluxon mode behavior.

Abstract

Modulation of the critical current across layers, Ic(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 Ic(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 Ic(H) patterns and multiple maxima in the Ic probability distribution were observed for Bi2Sr2CaCu2O8+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.