Filamentary flow of vortices in a Bi 2 Sr 2 Ca 1 Cu 2 O 8 + δ single crystal

Using microbridge technique, we have studied the vortex dynamics in a very low-temperature region (i.e., $T/{T}_{\text{c}}\ensuremath{\rightarrow}0$) of the B-T phase diagram of ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{Ca}}_{1}{\text{Cu}}_{2}{\text{O}}_{8+\ensuremath{\delta}}$ single crystal. We distinguish two types of vortex dynamics near the depinning threshold depending on the magnitude of the vortex-vortex interactions. For $0.01\ensuremath{\le}{\ensuremath{\mu}}_{0}Hl1\text{ }\text{T}$, we show that current-voltage $(I\text{\ensuremath{-}}V)$ characteristics are strongly dependent on the history of magnetic field and current cycling. The sharp peak, so-called ``peak effect,'' observed in ${\ensuremath{\mu}}_{0}H\text{\ensuremath{-}}{I}_{\text{c}}$ curve is due to a metastable state that can be removed after current cycling. At low field, $I\text{\ensuremath{-}}V$ curves show steps that would be clearly related to ``fingerprint phenomenon'' since the relationship ${R}_{\text{d}}=dV/dI$ exists. This can be attributed to vortices flow through uncorrelated channels for the highly defective lattice. Indeed, as field sufficiently increases, these peaks merge to make broader ones indicating a crossover from filamentary strings to braid riverlike in which vortex-vortex interactions becomes significant. As confirmed by the discontinuity in the critical exponent value $\ensuremath{\beta}$ determined in the vicinity of the threshold current using the power-law scaling $V\ensuremath{\sim}{(I\ensuremath{-}{I}_{\text{c}})}^{\ensuremath{\beta}}$ with a crossover from $\ensuremath{\beta}=2.2$ to $\ensuremath{\beta}=1.2$. The strong vortex correlation along the $c$ axis has been clearly demonstrated using the dc-flux-transformer geometry for transport measurements that confirms the pseudo-two-dimensional (2D) behavior of the flux-line lattice. Our transport studies are in good agreement with simulations results of 2D elastic objects driven by repulsive interactions through a random pinning potential.