AC response of vortices subjected to pinning by columnar defects in Tl 2Ba 2CaCu 2O 8 epitaxial films

Abstract

A micro-Hall-probe-based ac susceptometer is used to study the dynamics of vortices in Tl 2Ba 2CaCu 2O 8 (Tl-2212) epitaxial films with and without correlated disorder in a low field (<6 kG) and high temperature ( T>60 K) regime. The Bean critical state model for a thin superconducting disc in perpendicular magnetic field is used to extract the critical current density of the films. The temperature and frequency dependence of the irreversibility field ( B irr) is compared for samples with and without the linear defects. The B irr( T) curve shows a distinct discontinuity at B≈ B φ . The frequency dependence of the screening current density J( ω), extracted from the inphase component ( T ′ H) of the fundamental transmittivity, has been analyzed in the framework of Bose glass dynamics. This analysis shows that the vortex depinning temperature is ≈24 K for this Tl-2212 film and the depinning process starts via excitations of half-loops. The angle dependent studies of the susceptibility further suggest that these half-loops consist of many pancake vortices.