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

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