Anomalous Hall effect near half-filling in twisted bilayer graphene*

Magic-angle twisted bilayer graphene (tBLG) has been studied extensively owing to its exotic symmetry-broken phases, correlated Chern insulators, and unconventional superconductivity. Unambiguously identifying the isospin ordering of the various correlated ground states has so far been challenging owing to the large number of possible candidates, including fully spin and/or valley polarized states, intervalley coherent (IVC) states, nematics, and more. Although some devices exhibit an anomalous Hall effect (AHE) at ν = 1 or 3, so far no signatures of time-reversal symmetry (TRS) breaking have been reported in pristine tBLG devices at half-filling (ν = ±2). Here, we will discuss measurements of two tBLG devices with twist angles slightly away from the magic angle (0.95° and 1.2°) in which we observe an AHE at ν = +2 and -2, respectively. Our observations imply the existence of a TRS-broken ground state with a finite orbital magnetic moment that closely competes with the typically observed topologically trivial correlated insulating state at half-filling. Notably, we do not observe superconductivity upon doping away from half-filling in either of our devices, motivating the possibility that the ground state ordering we observe is incompatible with pairing.