Orbital Chern insulators at integer and half-integer fillings of a moiré superlattice

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 QAH states that emerge in twisted bilayer and twisted monolayer-bilayer (tMBG) graphene systems. 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 [1]. Finally, I will also present recent studies of the OCIs that emerge at ν=3/2 and 7/2 filling of the moiré superlattice unit cell in tMBG [2]. At ν=7/2, we observe a strong anomalous Hall effect with a jump of △R_xy≈1.2h/e² and a Streda formula behavior consistent with C=1,  all of which is strong evidence for the underlying QAH state. Our observation of Chern insulators at half-integer superlattice fillings suggests a spontaneous doubling of the superlattice unit cell, in addition to spin- and valley-ferromagnetism. This is confirmed by Hartree-Fock calculations, which find a topological charge density wave ground state at half-filling of the underlying C=2 band.

[1] H. Polshyn et al., Nature 588, 66–70 (2020).

[2] H. Polshyn et al., Nat. Phys., (in press), arXiv:2104.01178.