Superconductivity induced by spin-orbit coupling in a two-valley ferromagnet

Abstract: We analyze the origin of superconductivity in a ferromagnetically ordered state of multi-layer graphene systems placed in proximity to WSe$_2$. We model these materials by a two-valley system of interacting fermions with small pockets and Ising spin-orbit coupling. The model yields a canted ferromagnetic order, which gives rise to a half-metal. We obtain the magnon spectrum and derive two sets of magnon-mediated 4-fermion interactions: spin-flip interactions mediated by a single magnon and spin-preserving interactions mediated by two magnons. We argue that both processes have to be included on equal footing into the magnon-mediated pairing interaction between low-energy fermions from the filled bands. Then the full magnon-mediated interaction satisfies Adler criterion and for a valley-odd/spatially-even order parameter contains a universal attractive piece. This term is induced by spin-orbit coupling and is confined to energies which are parametrically smaller than the Fermi energy. We argue that, due to retardation, this magnon-mediated attraction gives rise to superconductivity despite that there exists a stronger static repulsion, in close analogy with how phonon-mediated attraction gives rise to pairing in the presence of stronger Coulomb (Hubbard) repulsion.