Engineering topological phases in graphene moiré heterostructures

Realizing topological phases at zero magnetic field has been a longstanding goal since Haldane’s theoretical proposal of the quantum anomalous Hall (QAH) state. My talk will focus on newly discovered QAH states that emerge in twisted bilayer and twisted monolayer-bilayer graphene (tMBG). In contrast to magnetically doped topological insulators, the QAH states in these moiré systems are driven by intrinsic strong interactions, which polarize the electrons into a single moiré miniband with Chern number of C = 1 or 2. Remarkably, the magnetization of these “orbital Chern insulators” (OCI) arises predominantly from the orbital motion of the electrons rather than the electron spin. I will discuss a novel effect originating from the curious magnetic properties of OCIs that enables non-volatile electrical switching of the magnetic and topological orders. Finally, I will present recent studies of the OCIs that emerge at half-fillings of the moiré superlattice unit cell in tMBG. Our observation suggests a topological charge density wave ground state that in addition to spin- and valley-ferromagnetism also spontaneously breaks the moiré superlattice symmetry.

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