CURRENT-DRIVEN VORTEX DYNAMICS IN UNTWINNED YBA2CU3O7 SUPERCONDUCTINGSINGLE-CRYSTALS

Current-driven vortex dynamics of type-II superconductors in the weak- pinning limit is investigated by quantitatively studying the current-d ependent vortex dissipation of an untwinned YBa2Cu3O7 single crystal. For applied current densities (J) substantially larger than the critic al current density (J(c)), non-linear resistive peaks appear below the thermodynamic first-order vortex-lattice melting transition temperatu re (T-M), in contrast to the resistive hysteresis in the low-current l imit (J < J(c)). These resistive peaks are quantitatively analysed in terms of the current-driven coherent and plastic motion of vortex bund les in the vortex-solid phase, and the non-linear current-voltage char acteristics are found to be consistent with the collective flux-creep model. The effects of high-density random point defects on the vortex dynamics are also investigated via proton irradiation of the same sing le crystal. Neither resistive hysteresis at low currents nor peak effe cts at high currents are found after the irradiation. Furthermore, the current-voltage characteristics within the instrumental resolution be come completely ohmic over a wide range of currents and temperatures, despite theoretical predictions of much larger J(c)-values for the giv en experimental variables. This finding suggests that the vortex-glass phase, a theoretically proposed low-temperature vortex state which is stabilized by point disorder and has a vanishing resistivity, may bec ome unstable under applied currents significantly smaller than the the oretically predicted J(c). More investigation appears necessary in ord er to resolve this puzzling issue.