Boran Zhou, Johns Hopkins University

The recent experimental studies of twisted bilayer graphene (TBG) raise a fundamental question: how do we understand Mott localization in a topological band? In this work, we offer a new perspective of Mott physics, which can be generalized to TBG directly in momentum space. In our theory, the Mott gap is understood as from an exciton-like hybridization Φ(k)c^\dagger(k)ψ(k) between the physical electron c and an ancilla fermion ψ. In the conventional Mott insulator of trivial band, the hybridization is s-wave with Φ(k) = U/2, where U is the on-site Hubbard interaction. On the other hand, the band topology in TBG enforces a topological Mott hybridization with Φ(k)~kx ± iky in a small region around k = 0. We dub this new Mott state as topological Mott localization because of the p±ip order parameter analogous to the topological superconductor. At ν = 0, we find a topological Mott semimetal with a low energy effective theory resembling that of the untwisted bilayer graphene. For ν = ±1,±2,±3, we show transitions from correlated insulators to Mott semimetals at smaller U. In the most intriguing density region ν = −2−x, we propose a symmetric pseudogap metal at small x, which hosts a small Fermi surface and violates the perturbative Luttinger theorem. Interestingly, the quasiparticle is primarily formed by ancilla fermion, which we interpret as a composite fermion formed by a hole bound to a particle-hole pair. Our theory offers a unified language to describe the Mott localization in both trivial and topological bands in momentum space, and we anticipate applications in other moir´e systems with topological Wannier obstruction, such as the twisted transition-metal dichalcogenide (TMD) homobilayer.