Pairing in magic-angle twisted bilayer graphene: role of phonon and plasmon umklapp

Identifying the microscopic mechanism for superconductivity in magic-angle twisted bilayer graphene (MATBG) is an outstanding open problem. While MATBG exhibits a rich phase-diagram, driven partly by the strong interactions relative to the electronic bandwidth, its single-particle properties, such as a non-trivial structure of the underlying Bloch wavefunctions, are unique and likely play an important role in its phenomenological complexity. We perform a theoretical study of the cooperative effects due to phonons and plasmons on pairing to disentangle their role played on superconductivity. We show that umklapp processes involving mini-optical phonon modes, arising as a result of the folding of the graphene acoustic branch due to the superlattice structure, contribute significantly towards enhancing pairing. We also investigate the role played by the dynamics of the screened Coulomb interaction on pairing, which leads to an enhancement in a narrow window of fillings and study the effect of external screening due to a metallic gate on superconductivity. We propose a smoking-gun experiment to detect resonant features associated with the phonon-umklapp processes in the differential conductance and also discuss experimental implications of a pairing mechanism relying on plasmons.