Asymmetry between flux penetration and flux expulsion in Tl-2212 superconductors

dc magnetic hysteresis as well as flux penetration and flux expulsion were investigated in Tl_2-yBa₂CaCu₂O_8-x polycrystals and monocrystals. All measurements were performed at 35 K and in the 0-5 T field range. Hysteresis measurements revealed an irreversibility field of about 2 T. Existing models predict identical field-cooled (fc) and zero-field-cooled (zfc) magnetizations and vanishing time dependence above this field. Although the identical fc and zfc magnetizations are in fact observed, the time dependence vanishes only for flux penetration after zero-field cooling; a remanence is preserved after field cooling and decays with a finite relaxation rate. Activation energies calculated on the basis of the thermal activation model display a pronounced field dependence, and are lower for flux penetration than for flux expulsion in high fields (H≥3 T) for all orientations. This behavior of extreme layered superconductors contradicts classical theoretical models and questions the original definition of the irreversibility line as well. All of our results are consistent with the recent theory of lock-in transition, and can be well interpreted by using those principles.