ac response of Hg-based superconductors: Surface barrier and bulk pinning contributions

A complete study of the ac response of Hg-based superconductors with 1, 2, and 3 Cu-O planes is presented. Under a dc field, the ac susceptibility χ_ac(T) displays a double transition, which is argued to be of intrinsic nature. The systematic analysis of the evolution of χ_ac (fundamental and higher harmonics) as a function of the experimental parameters, and the comparison with resistance and dc magnetization measurements, suggest that the higher temperature transition is ascribed to magnetic irreversibility due to surface barriers; the lower temperature transition is associated to bulk pinning.