Orbital magnetism and topology in moiré heterostructure

In moire narrow band systems, periodic interlayer interactions can be used to engineer isolated superlattice bands in which the Coulomb interaction is comparable to the bandwidth. As in a Landau level, this leads to electron interaction dominated physics despite the low density of electrons, enabling full density control using electrostatic gates. Unlike Landau levels, however, moire flat bands occur under time reversal symmetric conditions. I will discuss experiments that observe the spontaneous breaking of time reversal symmetry in twisted bilayer graphene, observed as magnetic hysteresis. Remarkably, hysteresis is accompanied by a quantized anomalous Hall effect, persisting to zero magnetic field and elevated temperatures. I will discuss the origins of this effect in the spontaneous orbital polarization of the electron system into a single, topological nontrivial superlattice band, and show magnetic imaging data acquired using nanoSQUID on tip microscopy that confirms the orbital character of the magnetism and reveals mesoscopic domain dynamics.