Comparison of Josephson fluxon modes in high- and low-temperature superconducting stacked Josephson junctions

We experimentally study and compare the perpendicular (c-axis) transport properties of layered high-Tc superconductor (HTSC) Bi₂Sr₂CaCu₂O₈₊ₓ and low-Tc (LTSC) Nb/Cu multilayers. Both for HTSC and LTSC samples similar anomalous features were observed. (i) The c-axis critical current Ic is multiple valued as a function of temperature T and in-plane magnetic field H. (ii) Ic exhibits extremely large fluctuations and the probability distribution P(Ic) has multiple maxima. (iii) The Ic(H) patterns are aperiodic. (iv) In the dynamic state, the flux-flow branches in current-voltage characteristics consist of multiple closely spaced sub-branches and exhibit large fluctuations due to switching between the sub-branches. Experimental data are in qualitative agreement with numerical simulations for the stack of long strongly coupled Josephson junctions. All this is taken as an evidence for the existence of multiple quasiequilibrium Josephson fluxon modes in our samples. From the study of LTSC multilayers we have found one-to-one correspondence between the anomalous behavior and appearance of stacked Josephson junctions in the two-dimensional state.