Abrikosov vortex escape from a columnar defect as a topological electronic transition in a vortex core

The microscopic scenario of vortex escape from a columnar defect under the influence of a transport current has been studied. For defect radii smaller than the superconducting coherence length the depinning process is shown to be a consequence of two subsequent topological electronic transitions in a trapped vortex core. The first transition at a critical current j _L is associated with the opening of Fermi surface segments corresponding to the creation of a vortex-antivortex pair bound to the defect. The second transition at a certain current j _d > j _L is caused by merging of different Fermi surface segments, which accompanies the formation of a freely moving vortex.