TRANSLATIONAL CORRELATIONS IN THE VORTEX ARRAY AT THE SURFACE OF A TYPE-II SUPERCONDUCTOR

We discuss the statistical mechanics of magnetic flux lines in a finit e-thickness slab of type-II superconductor. The long-wavelength proper ties of a flux-line liquid in a slab geometry are described by a hydro dynamic free energy that incorporates the boundary conditions on the f lux lines at the sample's surface as a surface contribution to the fre e energy. Bulk and surface weak disorder are modeled via Gaussian impu rity potentials. This free energy is used to evaluate the two-dimensio nal structure factor of the flux-line tips at the sample surface. We f ind that surface interaction always dominates in determining the decay of translational correlations in the asymptotic long-wavelength limit . On the other hand, such large length scales have not been probed by the decoration experiments. Our results indicate that the translationa l correlations extracted from the analysis of the Bitter patterns are indeed representative of behavior of flux lines in the bulk.