Multiple flux-flow branches and phase transition of Josephson fluxon lattice in intrinsic Bi2Sr2CaCu2O8+x stacked Josephson junctions.

We experimentally study the c-asis current-voltage characteristics (IVC's) of small area Bi2Sr2CaCu2O8+x mesas containing only a few intrinsic stacked Josephson junctions (SJJ's). In a magnetic field, H, parallel to the ab-pl ane, a Josephson flux-flow branch (FFB) appears in the IVC's. The FFB consi sts of multiple closely spaced I,ut distinct sub-branches and exhibit a wel l defined Fiske step structure. When normalised to the applied magnetic fie ld, the flux-flow branches for different H collapse into two universal curv es representing two different Josephson flux-flow regimes characterised by differ ent propagation velocities. A transition between the two regimes occ urs within a narrow field interval and may indicate phase transition of the Josephson fluxon lattice in intrinsic sJJ's.