Vortex flow characteristics of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ Long Intrinsic Josephson Junctions

Intrinsic Josephson junction characters in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$, where the junction length $L$ is much longer than the Josephson length \textit{$\lambda $}$_{J}$=\textit{$\gamma $s}, have been studied from view point of Josephson vortex dynamics, since they are expected to be released from the strong geometrical constraint, which confines Josephson vortices into a square box, and as a result they are free to move in the two dimensional channels. We fabricated such long junctions with $L$=20-40 $\mu $m and have measured $c$-axis transport characteristics. We have found that Josephson flow resistance $R_{f}$ suddenly suppressed at the intermediate field region of 1-2 T, then begins to reappear gradually as field is increased. The sharp lock-in transition as a function of angle appearing below it where the periodic oscillation of $R_{f}$ is seen, becomes immediately broad and a round peak above it, where the periodic oscillation of $R_{f}$ is no longer observable. It seems that the transition field becomes lower as $L$ is increased so that in short junctions it cannot be observed in a field region up to 6 T.