Flux droplet formation in NbSe2 single crystals observed by decoration

A potential barrier of geometrical origin, characterized by a barrier field Hp, governs vortex penetration in thin superconducting samples. We observed this effect by the decoration of crystals of NbSe2. Upon zero field cooling and applying a field Ha<Hp5 mT, vortices form a belt at the sample edges. At HaHp flux begins to penetrate in the form of dropletlike intrusions. The orientational correlation length for the vortex pattern inside the "droplets" is much larger than for the field-cooled patterns. This we interpret as recrystallization of the moving flux line lattice.