Critical Behavior Of Plastic Depinning Of Vortex Lattices In Two Dimensions: Molecular Dynamics Simulations

Using molecular dynamics simulations, we report a study of the dynamics of two-dimensional vortex lattices driven over a disordered medium. In strong disorder, when topological order is lost, we show that the depinning transition is analogous to a second-order critical transition: the velocity-force response at the onset of motion is continuous and characterized by critical exponents. Combining studies at zero and nonzero temperature and using a scaling analysis, two critical exponents are evaluated. We find v similar to (F-F-c)(beta) with beta = 1.3 +/- 0.1 at T = 0 and F > F-c, and v similar to T-1/delta with delta(-1) = 0.75 +/- 0.1 at F = F-c, where F-c is the critical driving force at which the lattice goes from a pinned state to a sliding one. Both critical exponents and the scaling function are found to exhibit universality with regard to the pinning strength and different disorder realizations. Furthermore, the dynamics is shown to be chaotic in the whole critical region.