Moiré Band Topology in Twisted Bilayer Graphene.

Recently twisted bilayer graphene (t-BLG) has emerged as a strongly correlated physical platform. Besides the apparent significance of band flatness, band topology may be another critical element in t-BLG and yet receives much less attention. Here we report the compelling evidence for nontrivial noninteracting Moiré band topology in t-BLG through a systematic nonlocal transport study and a K-theory examination. The nontrivial topology manifests itself as two pronounced nonlocal responses in the electron and hole superlattice gaps. We show that the nonlocal responses are robust to the twist angle and edge termination, exhibiting a universal scaling law. We elucidate that, although Berry curvature is symmetry-trivialized, two nontrivial Z2 invariants characterize the Moiré Dirac bands, validating the topological origin of the observed nonlocal responses. Our findings not only provide a new perspective for understanding the strongly correlated t-BLG but also suggest a potential strategy to achieve topological metamaterials from trivial vdW materials.