Dip effect in ac susceptibility due to surface barrier with flux creep

A model is proposed to describe the effect of surface barrier (SB) on ac susceptibility (ACS) and a different kind of dip effect (DE) in ACS is observed. Simulation based on this model with flux creep reveals two dips in ACS curve, one at temperature T_d^′ in real part χ^′ and the other at temperature T_d^″ in imaginary part χ^″. These two dips are different from the ones resulting from the peak effect in critical current density j_c, where the dips in χ^′ and χ^″ occur at the same temperature T_p. The DE is also characterized by a large χ^″ and a large |χ^′(T_d^′)|. The ACS curves for single crystals Bi₂Sr₂CaCu₂O₈ have been observed and compared with the DE in YBa₂Cu₃O₇ samples, confirming the numerical results. It is also shown that when flux creep is absent only kinks appear in χ^′ and χ^″ for a sample with SB.