Multiple flux-flow branches and phase transition of Josephson fluxon lattice in intrinsic Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+x/ stacked Josephson junctions

We experimentally study the c-axis current-voltage characteristics (IVCs) of small area Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+x/ mesas containing only a few intrinsic stacked Josephson junctions (SJJs). In a magnetic field, H, parallel to the ab-plane, a Josephson flux-flow branch (FFB) appears in the IVCs. The FFB consists of multiple closely spaced but distinct sub-branches and exhibit a well defined Fiske step structure. When normalised to the applied magnetic field, the flux-flow branches for different H collapse into two universal curves representing two different Josephson flux-flow regimes characterised by different propagation velocities. A transition between the two regimes occurs within a narrow field interval and may indicate phase transition of the Josephson fluxon lattice in intrinsic SJJs.