Anomalous Hall effect at half filling in twisted bilayer graphene

Magic-angle twisted bilayer graphene (MA-tBLG) hosts a wealth of symmetry-broken states arising due to strong Coulomb interactions in the moiré flat bands. Orbital magnetic states, with spontaneously broken time-reversal symmetry at zero field, have been previously reported at odd integer filling factors (ν = +1 and +3) in a small number of devices, manifesting as an anomalous Hall effect (AHE). Here, we will discuss two tBLG devices with twist angles slightly away from the magic angle (0.96° and 1.2°). Surprisingly, both devices display an AHE at half filling (ν = +2 and -2, respectively), which has not been observed previously. These states are not anticipated in existing theoretical models owing to competing intervalley-coherent, spin-polarized, and/or valley Hall states with lower energy. We suggest that our observations at half filling likely correspond to valley polarized phases stabilized by the combination of the increased bandwidth away from the magic angle and symmetry-breaking substrate potentials from the boron nitride. We do not observe superconductivity in our devices, hinting at a possible antagonistic relationship between the two. Our results motivate further investigation of tBLG away from the strongly coupled limit.