Berezinskii-Kosterlitz-Thouless transitions in spin-triplet superconductor

As the spin-triplet superconductivity arises from the condensation of spinful Cooper pairs, spin ordering is an essential feature. Hence the possibility arises for the Berezinskii-Kosterlitz-Thouless (BKT) phase transitions in not only the charge sector but also the spin sector in a 2d easy-plane spin-triplet superconductivity. However, it turns out that there are actually three possible BKT transitions, involving the unbinding of, respectively, the conventional vortices, the merons and the half-quantum vortices with vorticity in both the charge and the spin current. We show how all the transitions can be characterized by the relation between the voltage drop and the spin-polarized current bias and each phase by its characteristic Josephson coupling.