Magnetic-flux patterns on the surface of a type-II superconductor

Various regimes exist where the pattern of magnetic flux through the surface of a type-II superconductor is predominantly determined by surface effects and not by the pattern of flux in the bulk. In particular, for zero pinning and small enough flux density, the flux lattice at a flat surface is isomorphic to the classical two-dimensional electron lattice and crystallizes at a much higher temperature than does the bulk flux lattice. Within uniaxially anisotropic London theory, the flux-line energy recently calculated by Sudbo/ and Brandt shows that for sufficiently strong anisotropy of the sign appropriate for layered superconductors an isolated straight flux line in the bulk is only stable when it is either nearly parallel or nearly perpendicular to the layers. The ''flux chains'' observed on BSCCO by Bolle et al. occur in a regime where both orientations of stable flux lines should be present and thus reflect the interactions between parallel and nearly perpendicular flux lines.