Vortex entry conditions in typeII superconductors. Effect of surface defects.pdf

Vortex entry conditions in type-II superconductors. Effect of surface defects. D.Yu.Vodolazov 1, I.L.Maksimov 1, E.H.Brandt 2 1 - Nizhny Novgorod University, 603600, Nizhny Novgorod 2 - Max-Planck-Institut f¨ur Metallforschung, D-70506 Stuttgart, Germany 1 0 The conditions for the entry of vortices into type-II superconductors being in the Meissner and/or 0 2 mixed state, are studied by both numerical and analytical solution of the Ginzburg-Landau equa- tions. A modulation instability of the superconducting order parameter is shown to occur when the n a kinematic momentum (or supervelocity) of the condensate reaches a threshold value at the super- J conductor edge or surface. Due to this instability, vortices start to nucleate at the edge or surface 6 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 n curve. Based on these results a simple phenomenological model of the edge or surface barrier, taking o into account the effect of surface defects, is suggested. c - r p u I. INTRODUCTION s . t a Since the discovery of type-II superconductors until now, much interest was devoted to the effects that surface or m edge barriers have on the magnetic and dissipative properties of these materials. The solution of the barrier problem is d- closely connected with the condition for vortex entry into type-II superconductors. The problem of vortex p