Spin susceptibility of nonunitary spin-triplet superconductors

The description of unconventional superconductors relies on the identification of their pairing symmetry. In centrosymmetric crystals, pairs have either a spin-singlet or spin-triplet wave function. The most general spin triplet phase is nonunitary, and originates from unequal contributions of opposite spins. As a consequence, time-reversal symmetry is broken and the excitation spectrum consists of two non-degenerate bands. Such pairing may occur in chiral superconductors with strong spin-orbit coupling, and is suspected in a number of compounds. The spin susceptibility is an important probe to characterize the symmetry of the order parameter, and can be inferred from Knight shift or polarized neutron scattering measurements. Surprisingly it has not been calculated for nonunitary triplet phases. Our study fills this gap by providing a general formula for the spin susceptibility of nonmagnetic single-band centrosymmetric triplet superconductors with strong spin-orbit coupling. We apply it to group theoretically admissible nonunitary phases of specific crystal symmetries in the presence of strong spin-orbit coupling. We discuss implications for various experimental studies. Our results pave the way for further investigation of nonunitary triplet superconductivity in more complex systems.