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Orbital susceptibility and magnetic switching in twisted moiré heterostructures Egan, Shannon


Twisted moiré materials have opened up a new platform for realizing the quantum anomalous Hall (QAH) effect, with experiments reporting its observation in both twisted graphene and transition metal dichalcogenide heterostructures. A fascinating feature of these moiré QAH devices is the ability to switch the direction of Hall resistivity with very small magnetic fields or currents, making them a candidate for stable and programmable magnetic memory. Describing this phenomenon theoretically requires knowledge of both the orbital magnetization and orbital susceptibility, of which the latter is less well understood. This motivated us to derive a numerical formula for the zero-field orbital susceptibility of a general multi-band moiré continuum model, expressed in terms of quantities which reflect the geometry of the moiré bands. We use this formula to study the conditions for which magnetic switching can occur in moiré bands of twisted homobilayer MoS₂. We predict that the conduction band states of twisted MoS₂ give rise to two pairs of flat Chern bands in each valley: |C| = ±1 bands generated by moiré potential alone; and |C| = ±2 bands generated by a combination of moiré potential and spin-orbit coupling. All of the flat bands carry a large orbital magnetization ~1 Bohr magneton per moiré unit cell. Our Hartree-Fock analysis shows that at 1/2-filling, interactions generically drive the system into a spontaneous time-reversal broken valley-polarized state, yielding an "orbital Chern insulator" with quantized Hall resistivity. By calculating the Fermi sea contribution to orbital susceptibility, we show that the paramagnetism necessary to induce magnetic switching in the orbital Chern insulator state is confined to the gap between the |C| = ±2 Chern bands. The chemical potential and temperature dependence of the orbital susceptibility may help explain outstanding puzzles in the behaviour of moiré quantum Hall devices.

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