Correlation-induced valley splitting and orbital magnetism in a strain-induced zero-energy flatband in twisted bilayer graphene near the magic angle

In the vicinity of the magic angle in twisted bilayer graphene (TBG), many exotic correlated states, such as superconductivity, ferromagnetism, and topological phases, are observed because the two low-energy Van Hove singularities (VHSs) become exceedingly narrow, i.e., become flatbands. Heterostrain, strain in van der Waals structures where each layer is strained independently, can modify the single-particle band structure of the TBG and lead to various properties. Here, we show that heterostrain in a TBG near the magic angle generates a zero-energy flatband between the two VHSs. Doping the TBG to partially fill the zero-energy flatband, we observe a correlation-induced valley-polarized gap of ∼10 meV. By applying perpendicular magnetic fields, a large and linear response of the gap to magnetic fields is observed, attributing to the large orbital magnetic moments in TBG. The measured orbital magnetic moment per moiré unit cell is ∼15μB in the TBG, which is well consistent with our calculations.