Dimensionality of collective pinning in 2H-NbSe2 single crystals

ac susceptibility measurements have been used to determine the dimensionality of the collective pinning in 2H-NbSe2 crystals. We have analyzed the thickness dependence of the critical current versus field [J(c)(H)] curves for thicknesses between 6 and 166 mu m. Down to 15 mu m J(c)(H) is independent of the thickness showing that the pinning is three dimensional. This is in agreement with estimates from collective pinning theory. Deviations occur for the 6 mu m thick sample near the peak-effect regime, possibly indicating a crossover to two-dimensional behavior. In the thicker samples the peak effect clearly cannot be assigned to a dimensional crossover. The frequency dependence reflects a crossover from a Campbell regime to a nonlinear regime related to small flux creep effects.