Linear response in the AC susceptibility of vortices in YBa2Cu3O7 crystals with columnar defects

We explore the AC magnetic susceptibility of YBa2Cu3O7 single crystals with columnar defects. For vortices parallel to the defects the linear response is mainly determined by the Campbell penetration depth with a field independent Labusch parameter indicating that vortices are individually pinned. The small dissipation in this limit is likely to arise from thermal fluctuations of vortex segments between pairs of metastable states (two-level systems) in the Bose glass phase. Tilting vortices away from the tracks results in a reduction of the Bose glass temperature and an increase of the dissipation due to the reduction of the pinning forces. We discuss the onset of nonlinear response as the AC field increases.