Quantum anomalous Hall effect and electric-field-induced topological phase transition in AB-stacked MoTe${}_2$/WSe${}_2$ moiré heterobilayers

Abstract

We propose a new mechanism to explain the unexpected quantum anomalous Hall (QAH) effect and the electric-field-induced Mott-to-QAH phase transition without charge gap closure in ABstacked MoTe2/WSe2 moiré heterobilayers. We suggest that a hole-occupied band carrying a nonzero Chern number can be generated by an intrinsic band inversion between two topologically distinct Dirac bands and a Coulomb-interaction-induced gap opening in the moiré band structure with broken time-reversal symmetry (TRS). The Dirac-band dispersion is induced by a pseudomagnetic field, which is originated from the Berry phase of Bloch electrons in the presence of a moiré potential. The TRS is broken by valley-polarized interlayer-exciton condensation under an out-ofplane electric field. The valley polarization can be demonstrated by using an excitonic Bose-Hubbard (BH) model and Berezinskii-Kosterlitz-Thouless (BKT) transition to transverse ferromagnetism. In low electric fields, the equilibrium state of the system is a Mott insulator state. At a certain electric field, the exciton condensate combining with the Chern band becomes a more stable state and the Mott-to-QAH transition occurs. Since the band inversion is intrinsic, there is no charge gap closure at the transition.

Valley-coherent quantum anomalous Hall state in AB-stacked MoTe2/WSe2 bilayers

“…In this study, we directly probe the magnetic properties of the QAH state in AB-stacked MoTe 2 /WSe 2 moiré bilayers by layer-resolved and helicity-resolved optical spectroscopy. We show that the holes are distributed in both layers, and contrary to the theoretical predictions [15][16][17][18][19][20][21] , the QAH ground state is not valley-polarized but is consistent with a valley-coherent state, i.e. hybridization of states from different layers and different valleys near the Fermi level, thus opening a correlated charge gap.…”

“…Such a state could occur, for instance, via an exciton condensation mechanism near band inversion 34,35 . The excitonic mechanism could be important here given the weak interlayer coupling in AB-stacked TMD bilayers 20,34,35 . Future experiments with an ultrathin spacer between the TMD bilayers that quenches the interlayer tunneling while maintaining the strong Coulomb interaction and reasonable moiré potential could test the validity of the mechanism.…”

“…1d), i.e. hybridization of states from different layers but from the same valley near the Fermi level [15][16][17][18][19][20] . The spin-polarized QAH state requires mixing states from different layers and different valleys; a correlated charge gap is opened at the Fermi level; the state is thus spontaneous layer-and valley-coherent 34,35 .…”

“…It was pointed out that valley Chern bands could still form from a strain pseudo-magnetic field 17 or from interlayer tunneling when the out-of-plane spin is no longer conserved 15 ; both can be induced by lattice reconstruction in moiré bilayers. These theories naturally predict a (fully or partially) valley-polarized QAH ground state [15][16][17][18][19][20][21] .…”

Intrinsic spin Hall torque in a moire Chern magnet

“…1b). This is consistent with previous results on AB-stacked MoTe 2 /WSe 2 bilayers [3], where a magnetic Chern insulator arises due to the interplay of strong Coulomb repulsion and underlying Berry curvature of the moiré subbands [19][20][21][22][23].…”