Anisotropy of Bi_1.5Pb_0.7Sr_1.8CaCu₂O_8+δ Single Crystal

Abstract

The c-axis anisotropy of Bi_1.5Pb_0.7Sr_1.8CaCu₂O_8+δ single crystal has been investigated by torque magnetometry. The torque curve of the Bi_1.5Pb_0.7Sr_1.8CaCu₂O_8+δ crystal is reversible except for θ _cb ≃ 90° under H = 8 kG and T = 77.3 K upon rotating around the a axis. The effective mass model of the torque yields the anisotropy parameter ${\gamma _{c}} = \sqrt {{{m_{c}}/{m_{b}}}}= 185\pm 6$. We conclude that the anisotropy parameter γ_c of the lead-doped crystal is not appreciably different from that of a lead-free crystal. The preferential accommodation of vortices within the planar defects has been explored thoroughly, but no evidence for the vortex lock-in state is found in the torque curves. We propose that linear correlated pinning centers along the CuO₂ plane are effectual in the Pb-doped crystal rather than planar pinning centers.