Jing-Yu Zhao, Ya-Hui Zhang
Understanding the mechanism of superconductivity in twisted bilayer graphene (TBG) is a central challenge in correlated moiré materials. The most intriguing question is about the nature of the normal state: is the Cooper pair formed from small Fermi surface or large Fermi surface? Here, we propose a unified theoretical framework describing a symmetric pseudogap metal with small hole pockets, dubbed the second Fermi liquid (sFL), which is topologically distinct from the conventional Fermi liquid. Our theory shows that superconductivity in TBG emerges from the sFL phase, with local moment pairing transferred to mobile carriers, giving rise to a smaller nodal superconducting gap on the small hole pockets. This framework simultaneously explains the pseudogap metal above the transition temperature and the two-gap nematic superconductivity below it, providing a comprehensive understanding of correlated physics in TBG.