AC susceptibility of pinned vortices in YBa2Cu3O7 crystals with correlated disorder

L. CIVALE; G. PASQUINI; P. LEVY; G. NIEVA; H. LANZA

Simposio; 8th International Symposium on Superconductivity (ISS '95); 1995

We explore the ac susceptibility of YBCO single crystals with columnar defects in the limit of high temperature and low magnetic field. For low enough ac field we find an almost non-dissipate linear response that is mainly determined by the Campbell penetration depth. We conclude that vortices are individually pinned in this limit and that the small dissipation is likely to arise from thermal fluctuations in the Bose glass phase(two level systems). The onset of non linear response occurs when the amplitude of the vortex oscillations exceed a characteristic value smaller than the local radius of the pinning potential.