Structural studies of magnetic flux line lattices near critical transitions

Small angle neutron scattering (SANS) studies are an essential tool for studying vortices in the bulk of type II superconductors, providing deep understanding of their three dimensional microscopic structure in a wide range of fields and temperatures. This talk will summarize detailed studies of flux lattices in the vicinity of two critical transitions: (1) flux lattices in the vicinity of the critical current in 2H-NbSe2. We find a clear evidence for a two step depinning process: as a function of increasing driving force three regimes are observed - first, no motion; then disordered, plastic motion; and finally at high velocities a coherently moving crystal, (2) flux lattices in the vicinity of the peak effect, below the upper critical field, Hc2, in Nb. Our studies reveal drastic structural disordering, characterized by complete loss of positional and orientational correlations, whereas the lines remain well correlated along their length. (orig.)