Critical current densities and the effects of thermally assisted flux flow in single crystals of Bi₂Sr₂CaCu₂O₈

The authors have grown single crystals of Bi₂Sr ₂CaCu₂O₈ and measured their DC magnetisation (using both SQUID and Foner magnetometers) and AC susceptibility in small DC magnetic fields. They find a flux expulsion fraction of up to 34% of the shielding effect in fields down to 3 G, significantly larger than found for the YBa₂Cu₃O₇ crystals. Critical currents were determined as a function of temperature by applying Bean's model and are compared with the critical currents found for single-crystal YBa ₂Cu₃O₇. The critical current densities are strongly dependent on field and temperature. A dramatic frequency dependence of the AC susceptibility in a small DC magnetic field is also observed. They show that a model based upon thermally assisted flux flow (TAFF) accounts for the observed effects.