Two- and fourfold ab-plane torque symmetry in untwinned YBa2Cu3O7 single crystals

Reversible magnetic-torque measurements in untwinned, single-crystal YBa2Cu3O7 show both twofold and fourfold symmetry in the ab plane. Different c-axis anisotropy parameters are found from the London model: γca=√ (mc/ma) =7.55±0.63 and γcb=√ (mc/mb) =8.95±0.76 at 90 K in 0.8 T. The ab-anisotropy parameter is γab=√ (ma/mb) =1.18±0.14. The free energy of the fourfold symmetry has a minimum, and neointrinsic pinning is found in the in-plane irreversible magnetic torque, when the field is applied parallel to the a or b axis. These results are consistent with dx2-y2 symmetry of the bulk order parameter. copyright 1997 The American Physical Society