  At the nucleus of a superconducting vortex is a small, circular region where
superconductivity is destroyed. Like an atomic nucleus, this core may become
deformed, and such distortions can have important consequences in
non-equilibrium situations. Using Ginzburg-Landau theory, we have investigated
this phenomenon for vortices in the presence of artificial defects. We show
that when a vortex approaches the vicinity of a defect, an abrupt transition
occurs in which the vortex core develops a "string" extending to the defect
boundary, while simultaneously the supercurrents and associated magnetic flux
spread out and engulf the defect. The energetics of stretching the string
determines the pinning behavior of the vortex. Experimental consequences of
these strings are discussed.
