Spin-triplet superconductivity and ferromagnetism in moire systems: A case study for twisted double bilayer graphene

Recent experiments have observed possible ferromagnetism and spin-triplet superconductivity in twisted double bilayer graphene that comprises a pair of AB-stacked bilayer graphene under a displacement field. We show that ferromagnetism and spin-triplet pairing can naturally be the dominant instabilities in such systems as a general consequence of the valley-dependent van Hove physics when the nesting is weak. Specifically, by performing a renormalization group analysis for all possible pairing, magnetic, and charge instabilities, we find ferromagnetism or spin-triplet pairing to be dominant throughout a broad parameter space of interactions and low-energy band structure properties we consider. Finally, we discuss the possibility of topological superconductivity and relevance to the experimental findings.