Dynamics of two-dimensional pancake vortices in layered superconductors

The dynamics of two-dimensional (2D) pancake vortices in Josephson-coupled superconducting/normal-metal multilayers is considered within the time-dependent Ginzburg-Landau theory. For temperatures close to T-c a viscous drag force acting on a moving 2D vortex is shown to depend strongly on the conductivity of normal-metal layers. For a tilted vortex line consisting of 2D vortices the equation of viscous motion in the presence of a transport current parallel to the layers is obtained. The specific structure of the vortex line core leads to substantial deviations from the Bardeen-Stephen theory. The viscosity coefficient is found to depend essentially on the angle gamma between the magnetic field B and the c axis normal to the layers. For field orientations close to the layers the nonlinear effects in the vortex motion appear even for slowly moving vortex lines (when the in-plane transport current is much smaller than the Ginzburg-Landau critical current). In this nonlinear regime the viscosity coefficient depends logarithmically on the vortex velocity V.