Pairing Symmetry from In-plane Torque Anisotropy in Tl2201 Thin Films

The in-plane torque anisotropy of high-quality epitaxial Tl(2)Ba(2)CuO(6+delta) thin films patterned into disks has been investigated. We have found that the angular dependence of the reversible torque displays a clear fourfold symmetry added to a twofold one. Excluding all experimental artifacts due to geometry or magnetic-field misalignment, we propose that this is a real bulk evidence for a d+s pairing state allowed by the possible breaking of the tetragonal symmetry at low temperature. Identical measurements on a NbN thin film and a NbSe(2) single crystal show no
anisotropy, in agreement with the expected symmetry for these classical superconductors.