Time-Reversal Symmetry and the Structure of Superconducting Order Parameter of Nearly Ferromagnetic Spin-Triplet Superconductor UTe2

The space-group approach to the wavefunction of a Cooper pair is extended to Shubnikov groups of type III and applied to newly discovered nearly ferromagnetic superconductor UTe2. A nonunitary triplet SOP (superconducting order parameter), manifested experimentally, was obtained for Shubnikov group m’m’m . The nodal structure of triplet nonunitary SOP is investigated and it was shown that in the case of Shubnikov III symmetry nodal structure of ESP (equal spin pairing) state is in some cases unstable, i.e. changes when a complex phase winding is added. The irreducible corepresentations of a Cooper pair wave function corresponding to m’m’m symmetry and to all experimental data for UTe2 are identified.