Velocity Fluctuations of Vortices in Driven Two-Dimensional Vortex Matter

A molecular dynamic simulation is conducted for a dense two-dimensional vortex matter. At the critical moment when depinning events were concentrated and dynamic phase transition occurred, the simulation shows unexpectedly slow fluctuation in collective velocity of vortices along the direction of driving force (longitudinal direction). A detailed analysis exhibited heavy braiding of vortex flow-channels, and it was revealed that the velocity fluctuation is well synchronized with fluctuation of local density of flow-channels upon the depinning transition. In addition, vortex transport properties of a superconducting MgCNi3 single crystal seemed to show consistency between the experiment and the theory. At last, the density of flow-channels is proposed as a dynamic order parameter of current-driven vortex phase.