Cascade of zero-field Chern insulators in magic-angle bilayer graphene

The interplay between strong electron-electron interactions and symmetry breaking can have a profound influence on the topological properties of materials. In magic-angle twisted bilayer graphene (MATBG), the flat band with a single SU(4) flavor associated with the spin and valley degrees of freedom gains a non-zero Chern number when C _2z symmetry or C _2z T symmetry is broken. Electron-electron interactions can further lift the SU(4) degeneracy, leading to Chern insulator states. Here, we report a complete sequence of zero-field Chern insulators at all odd-integer fillings (ν = ±1, ±3) with different chiralities (C = 1 or -1) in hBN-aligned MATBG which structurally breaks C _2z symmetry. The Chern states at hole fillings (v = -1, -3), which are firstly observed in this work, host an opposite chirality compared with the electron filling scenario. Furthermore, at the valence band filling ν = -7/2, the zero-field symmetry broken Chern insulator with C = -1 can be observed. Remarkably, a prominent Streda-formula violation around the v = -3 state has been observed. By doping the Chern gap at v = -3 with a notable number of electrons at finite magnetic field, the Hall resistance R _yx robustly quantizes to ∼h/e ², whereas the longitudinal resistance R _xx vanishes, indicating that the chemical potential is pinned within a Chern gap, forming the magnetic field stabilized incommensurate Chern insulator states. By providing the first experimental observation of zero-field Chern insulators in the flat valence band, our work fills up the overall topological framework of MATBG with broken C _2z symmetry.