AC susceptibility of pinned vortices in Y Ba 2 Cu 3 O 7 crystals with correlated disorder

Abstract

We explore the ac magnetic susceptibility of YBa 2 Cu 3 O 7 single crystals with columnar defects in the limit of high temperatures and low magnetic fields. For low enough ac field we find an almost non-dissipative 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 nonlinear response occurs when the amplitude of the vortex oscillations exceeds a characteristic value smaller than the total radius of the pinning potential.