Weimin Chen — Page 3

arXiv1999

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, $ρ_f = 1.4(B-B^*)$. 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)).

cond-mat.supr-con↗

arXiv1999

Evidence of Structural Instability Near T*~250 K in the Resistivity of Bi_2Sr_2CaCu_2O_8 Whiskers

We report anomalous features in the normal-state resistivity of single crystalline Bi_2Sr_2CaCu_2O_8 whiskers near some characteristic temperature T* \~ 250 K. From these varied oxygen-doping samples, the resistance (R) was found to deviate {\it upward} from the linear-T relation for T< T*, while R keeps decreasing with T. The deviation, $ΔR \equiv R - (a + bT)$, reaches a maximum in a temperature range of ~ 25 K just before R-T follows the well-known descent associated with the pseudogap opening in the underdoped samples. A second kink was also found in some samples immediately following the first one, resulting in an S-shaped feature in the R-T curve. The resistance then resumes nearly linear-T decrease below the anomalies in contrast to the first case. We interpret these features as being related to crystal structure transformation (or lattice distortion), as was evidenced in thermal and elastic property measurements. These structural instabilities seem to be connected to the subsequent dynamics as T is further lowered.

cond-mat.supr-con↗