Origin of possible nonunitary pairing in Fe(Se,S) driving Bogoliubov Fermi surface

FeSe based materials have been extensively studied as hosts for exotic superconducting and topological phases. Of particular interest is the finding that Fe(Se, S) upon sulfur doping exhibits a nematic quantum critical point, which separates two distinct superconducting phases on its two sides at low temperature. One of the unusual aspects of the superconducting phases is the abrupt increase in the zero energy spectral weight across the nematic QCP. We have previously proposed a mean-field level model that gives rise to Bogoliubov Fermi surfaces and is consistent with the experimental finds near the QCP. The existence of Bogoliubov Fermi surfaces in our model relies on interband as well as non-unitary triplet pairing, but identifying a microscopic model giving rise to such a state has proven elusive. Here we discuss possible microscopic Hamiltonians that preserve time reversal symmetry but break it in the ground state of nonunitary triplet pairs.