Anisotropic transport critical current density and its field dependence in a Bi 2Sr 2CaCu 2O x single crystal

The authors observed an anisotropy in the transport critical current density, J_ct, at 77 K and 500 G, for a Bi ₂Sr ₂CaCu ₂O _x single crystal. The transport critical current density, J_ct, along the c-axis for a field parallel to the a-axis, with the Lorentz force parallel to the b-axis, was about three times larger than that for the field parallel to the b-axis, with the Lorentz force parallel to the a-axis. This shows the possibility that a bismuth atom modulation structure, existing only along the b-axis, may act as an effective pinning site. J_ct with the Lorentz force perpendicular to the BiO insulating layers was about 10 times larger than that with the force along the layers, indicating that the BiO layers also serve as pinning centres. A correlation was observed between the decrease in J_ct and the field dependence rate, with J_ct ∝ B^{- n} (where n = 0.8-1.3). This is attributed to the pronounced flux creep effect in this material.