Gap symmetry and intrinsic intraplane pinning of untwinned YBa2Cu3O7 single crystals

The interplane (cb) and intraplane (ab) anisotropy of untwinned YBa2Cu3O7 single crystals has been investigated by means of the torque magnetometry. To extract the reversible and irreversible components, the torque was measured as a function of increasing angle as well as decreasing angle. The interplane irreversible torque tau(irr) shows two-fold pearls at theta(c) = 90 degrees and 270 degrees (of half width 11 degrees at 77 K) due to the well-known intrinsic interplane pinning. A novel intrinsic pinning has been discovered in the intraplane irreversible torque, i.e., pi(irr) shows four-fold peaks of half width 20 degrees at 77 K when theta(alpha) = 0 degrees, 90 degrees, 180 degrees and 270 degrees. Ne argue that the intrinsic intraplane pinning comes from the four-fold nature of the gap parameter.