  Using micro-bridge technique, we have studied the vortex dynamics in a very
low temperature region (i.e. T/Tc -> 0) of the B-T phase diagram of
Bi_2Sr_2Ca_1Cu_2O_{8+\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 <= \mu_0H <= 1T, we show that
current-voltage characteristics (I-V) are strongly dependent on the history of
magnetic field and current cycling. The sharp peak, so called "peak effect"
(PE), observed in \mu_0H-Ic curve is due to a metastable state which can be
removed after current cycling. At low field, I-V curves exhibit steps which
clearly enligth a "fingerprint phenomenon" as it can be seen the current
dependence of the differential resistance Rd = dV/dI. We associate this to
vortices flow through uncorrelated channels for the highly defective lattice.
Indeed, as field sufficiently increase, these peaks merge giving broader ones
indicating a crossover from filamentaty strings to braid river like in which
vortex-vortex interactions becomes significant. As confirmed by the
discontinuity in the critical exponent value \beta determined in the vicinity
of the threshold current using the power-law scaling V (I-Ic)^\beta with a
crossover from \beta = 2.2 to \beta = 1.2. The strong vortex correlation along
the c-axis has been clearly demonstrated using the dc-flux-transformer geometry
for transport measurements which confirms the pseudo-2D behaviour of the FLL.
Our transport studies are in good agreement with recent simulations results of
2D elastic objects driven by repulsive interactions through a random pinning
potential.
