2- AND FOURFOLD AB-PLANE TORQUE SYMMETRY IN UNTWINNED YBA2CU3O7 SINGLE-CRYSTALS

Reversible magnetic-torque measurements in untwinned, single-crystal Y Ba2Cu3O7 show both twofold and fourfold symmetry in the ab plane. Diff erent c-axis anisotropy parameters are found from the London model: ga mma(ca) = root m(c)/m(a) = 7.55+/-0.63 and gamma(cb) = root m(c)/m(b) = 8.95+/-0.76 at 90 K in 0.8 T. The ab-anisotropy parameter is gamma(a b) = root m(a)/m(b) = 1.18+/-0.14. The free energy of the fourfold sym metry has a minimum, and neointrinsic pinning is found in the in-plane irreversible magnetic torque, when the field is applied parallel to t he a or b axis. These results are consistent with d(x2-y2) symmetry of the bulk order parameter. [S0163-1829(97)07341-4].