Chern insulators at odd fillings in magic-angle twisted bilayer graphene

The interplay of magnetic field and the moire superlattice in magic-angle twisted bilayer graphene provides a rich playground for correlated electron physics. We report on magneto-transport measurements of a twisted bilayer graphene device(1.17°±0.02°). At high out-of-plane magnetic fields, we observe Landau fans at integer fillings with well quantized Hall plateaus: σ_xy=νe²/h, corresponding to integer Chern numbers,ν∈Ζ. These are formally equivalent to the Chern bands of the Hofstadter butterfly formed by the single-particle spectrum of an electron in a periodic potential at high magnetic fields. At fillings of n/n₀=-1, we find ν=3, while at n/n₀=3, ν=1. Here, n is the total carrier density. n₀ is the carrier density corresponding to one electron per moire cell. For these fillings, we find that the field dependence of the Hall number n_H(B) extracted from the Hall resistivity, is non-monotonic indicating the opening of a gap at high fields(B>4T). This finding is confirmed by measuring the field dependence of the thermal activation gaps for both in plane and out of plane magnetic fields. The linear field dependence of the gaps being consistent with Zeeman splitting, suggests the emergence of a spin-aligned ferromagnetic phase.