INPLANE TORQUE AND GAP SYMMETRY OF UNTWINNED YBA2CU3O7 CRYSTALS

The reversible magnetic torque of untwinned YBa2Cu3O7 Single crystals shows the four-fold symmetry in the a-b plane. The irreversible torque indicates evidence for a novel intrinsic pinning along the a and b ax es. These facts mean that the free energy of the four-fold symmetry ha s a minimum when the field is applied along the a or b axis. The resul ts are consistent with those expected from the d(x2-y2) symmetry and r ule out the possibility of the d(xy) symmetry. The Fermi surface aniso tropy is not responsible for the observed anisotropy. This is first bu lk evidence for the k-dependent gap anisotropy on the Fermi surface. T he two-fold anisotropy parameter is found as gamma(ab) = root m(a)/m(b ) = 1.18 +/- 0.14.