GAP SYMMETRY AND INTRINSIC INTRAPLANE PINNING OF UNTWINNED YBA2CU3O7 SINGLE-CRYSTALS

The interplane (cb) and intraplane (ab) anisotropy of untwinned YBa2Cu 3O7 single crystals has been investigated by means of the torque magne tometry. To extract the reversible and irreversible components, the to rque was measured as a function of increasing angle as well as decreas ing angle. The interplane irreversible torque tau(irr) shows two-fold pearls at theta(c) = 90 degrees and 270 degrees (of half width 11 degr ees at 77 K) due to the well-known intrinsic interplane pinning. A nov el intrinsic pinning has been discovered in the intraplane irreversibl e 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 27 0 degrees. Ne argue that the intrinsic intraplane pinning comes from t he four-fold nature of the gap parameter.