Observation of a Universial Low Temperature Limit of the Free Flux Flow Resistivity in Superclean Bi_2Sr_2CaCu_2O_8 Whiskers

We report peculiar vortex motion in clean Bi$_2$Sr$_2$CaCu$_2$O$_8$ single crystals, which reveals nearly linear-$T$ resistivity tails extending to low temperatures. The resistivity was found to satisfy the classical Bardeen-Stephen model for free flux flow, which is otherwise hard to observe under normal experimental conditions in high-$T_c$ cuprates. Moreover, in the superclean regime, a universal temperature-independent value for the flux flow resistivity was observed, $\rho_f = 1.4(B-B^*)$. We interpret this carrier-relaxation {\it independent} behavior as being related to the dissipation caused by the minigap states within the vortex core. The results are in agreement with theoretical calculations (N. B. Kopnin and G. E. Volovik, Phys. Rev. Lett. {\bf 79}, 1377 (1997)).