Pairing symmetry from in-plane torque anisotropy in thin films

The in-plane torque anisotropy of high-quality epitaxial (Formula presented) 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 in the frame of a model by Beal-Monod and Maki that this is real bulk evidence for a (Formula presented) pairing state allowed by the possible breaking of the tetragonal symmetry at low temperature. Identical measurements on a NbN thin film and a (Formula presented) single crystal show no anisotropy, in agreement with the expected symmetry for these classical superconductors. © 1998 The American Physical Society.