Superconductivity classes in YBa2Cu3O7 and the orthorhombic distortion anomaly

The symmetry classification of the possible superconducting states is made for the high-T_c superconductor YBa₂Cu₃O₇ in assumption that the superconductivity is generated in the nearly tetragonal background of the layers of fivefold-coordinated Cu atoms and that the orthorhombic distortion may be considered as a perturbation. The interaction of the order parameter with the orthorhombic splitting b-a, which gives rise to the orthorhombic distortion anomaly at T_c as well as to the possibility of the additional second-order transition slightly below T_c is discussed.