Pairing symmetry in single-layer tetragonal Tl₂Ba₂CuO_6+δ from in-plane torque anisotropy

The in-plane torque anisotropy of the high-Tc cuprate Tl2Ba2CuO6+δ gives evidence of a mixed s+d pairing state allowed by the possible breaking of its tetragonal symmetry at low temperature. The validity of our result is supported by the absence of any in-plane anisotropy in the classical superconductors NbN and the layered 2H-NbSe2 measured under the same conditions.