Vortex entry conditions in type-II superconductors. Effect of surface defects

The conditions for the entry of vortices into type-II superconductors being in the Meissner and/or mixed state, are studied by both numerical and analytical solution of the Ginzburg-Landau equations. A modulation instability of the superconducting order parameter is shown to occur when the kinematic momentum (or supervelocity) of the condensate reaches a threshold value at the superconductor edge or surface. Due to this instability, vortices start to nucleate at the edge or surface and then penetrate deeper into the sample. It is found that the presence of surface defects causes a noticeable drop of the first penetration field and leads to a qualitative change of the magnetization curve. Based on these results a simple phenomenological model of the edge or surface barrier, taking into account the effect of surface defects, is suggested.