We study the phase transition between a trivial and a time-reversal-invariant topological supercon- ductor. By analyzing the interplay of symmetry, topology and energetics, we show that for a generic normal state band structure, the phase transition occurs via extended intermediate phases in which even- and odd-parity pairing components coexist. For inversion symmetric systems, the coexistence phase spontaneously breaks time-reversal symmetry. For noncentrosymmetric superconductors, the low-temperature intermediate phase is time-reversal breaking, while the high-temperature phase preserves time-reversal symmetry and has topologically protected line nodes. Furthermore, with approximate rotational invariance, the system has an emergent U(1)×U(1) symmetry, and novel topological defects, such as half vortices binding Majorana fermions, can exist. Relevance of our theory to superconducting pyrochlore oxide Cd2Re2O7 and half-Heusler materials is discussed.
YW, Cho, Hughes, and Fradkin, PRB 93, 134512 (2016),
YW and Fu, arXiv:1703.06880