Anisotropy of Bi_(1.5)Pb_(0.7)Sr_(1.8)CaCu_2O_(8+#delta#) Single Crystal

摘要

The c-axis anisotropy of Bi_(1.5)Pb_(0.7)Sr_(1.8)CaCu_2O_(8+#delta#) single crystal has been investigated by torque magnetometry. The torque curve of the Bi_(1.5)Pb_(0.7)Sr_(1.8)CaCu_2O_(8+#delta#) crystal is reversible except for #theta#_(cb)approx=90 deg 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 = root m_c/m_b=185+-6. We conclude that the anisotropy parameter #gamma#_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 CuO_2 plane are effectual in the Pb-doped crystal rather than planar pinning center.