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

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Abstract
The authors have grown single crystals of Bi2Sr2CaCu2O8 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 YBa2Cu3O7. 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.