Flux-Line Arrangement in Superconductors as Revealed by Direct Observation

The flux-line lattice in a type-II superconductor in the remanent state is investigated by means of a decoration method. The triangular lattice contains various kinds of defects, the most important of which are flux-line dislocations. A critical lattice parameter dc exists such that for lattice parameters d>dc the flux lines form no longer a lattice but a ``flux-line fluid.'' A belt directly underneath the specimen surface is free from flux lines. The transition from the ``fluid'' to the flux-free region has been utilized to estimate in a fairly direct way the pinning forces acting on the flux lines.