We report small-angle neutron scattering measurements on the vortex lattice in a PbIn polycrystal in the presence of an applied current. Using the rocking curves as a probe of the distribution of current in the sample, we observe that vortex pinning is due to the surface roughness. This leads to a surface current that persists in the flux-flow region. We show the influence of surface treatments on the distribution of this current.

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According to the Mathieu-Simon continuum theory of the vortex state, a large nondissipative supercurrent can flow over a small depth from a rough surface, up to an easily estimated critical value K c (A/m) of the surface current density. It is shown that K c must saturate at high fields in an anisotropic crystal when the surface roughness is increased, and the corresponding upper bound only depends on fundamental parameters of the material. Measurements in 2H-NbSe 2 crystals confirm this saturation effect quantitatively, as well as the proposed idea that, in a large class of soft samples, the critical current should be entirely accounted for by surface K c currents.