p-wave superconductivity and the axi-planar phase of triple-point fermions

Abstract

We consider weak-coupling superconductivity in the inversion- and rotation-symmetric system of two pseudospin $s=1$ low-energy fermions of opposite chirality. General contact interactions can lead to Cooper instabilities towards d-wave or towards a novel $2\times 3$ p-wave matrix order parameter. We compute the Ginzburg-Landau free energy for the latter. Remarkably, in this case the Ginzburg-Landau free energy can be minimized exactly, with the resulting ordered state being analogous to the ``axi-planar" p-wave, which exhibits extra degeneracy, and generally breaks time reversal symmetry. Whereas a generic Ginzburg-Landau free energy for our order parameter has only $U(1) \times SO(2) \times SO(3)$ symmetry, at weak coupling we find it displaying the enlarged $U(1)\times SO(3) \times U(1) \times SO(3)$ symmetry, broken down to $SO(2)\times SO(2)$ in the axi-planar ordered phase, and leading therefore to six Goldstone bosons. We show how the lattice, once restored, fixes the allowed values of the magnetization in the superconducting state by locking the spatial directions implicit in the order parameter to its high-symmetry axes.