Anisotropic thermally activated flux flow and flux flow in Bi2Sr2CaCu2Oy crystals

The magnetoresistance of two single crystals of Bi2Sr2CaCu2Oy was investigated in magnetic fields up to 1.6T with a direction angle, θ, between 0° and 180° with respect to the c-axis. The resistivity of the crystals was analyzed in terms of the interpolative expression 1/ϱ=1/ϱFF+1/ϱTAFF, where ϱFF is the flux flow resistivity and ϱTAFF is given by the thermally activated flux flow (TAFF) expression, ϱTAFF=2πϱFF(U/T)exp(-U/T) where T is the temperature and U the activation energy. Results indicated that H∝H-α, where α ≈ 13, for fields applied both perpendicular and parallel to the c=axis. The angular dependenc e U, was fit with a form of the anisotropic Ginzburg-Landau expression U ∝(cos2θ + ϵsin2θ)−ga2, where ϵ is the inverse of the mass anisotropy factor. Values of the mass anisotropy factor obtained for the two crystals were 700 and 300, respectively, and were confirmed by measuring the torque anisotropy. These results demonstrate that the activation barriers as obtained from an interpolative analysis of the TAFF and flux flow resistivity reflect the anisotropic superconducting nature of these materials.