Magic in twisted transition metal dichalcogenide bilayers

TL;DR

This paper investigates twisted WSe2 bilayers, revealing a magic angle where the valence band becomes flat, enabling potential realization of topological and correlated phases such as quantum Hall and Mott insulators.

Contribution

It identifies a specific magic angle in twisted WSe2 bilayers and predicts emergent topological and correlated electronic phases near this angle.

Findings

Discovery of a magic angle with nearly flat top valence band.

Prediction of topological flat bands and Mott insulators at specific fillings.

Potential for realizing fractional quantum anomalous Hall effect.

Abstract

The long wavelength moir\'e superlattices in twisted 2D structures have emerged as a highly tunable platform for strongly correlated electron physics. We study the moir\'e bands in twisted transition metal dichalcogenide homobilayers, focusing on WSe$_2$, at small twist angles using a combination of first principles density functional theory, continuum modeling, and Hartree-Fock approximation. We reveal the rich physics at small twist angles $\theta<4^\circ$, and identify a particular magic angle at which the top valence moir\'e band achieves almost perfect flatness. In the vicinity of this magic angle, we predict the realization of a generalized Kane-Mele model with a topological flat band, interaction-driven Haldane insulator, and Mott insulators at the filling of one hole per moir\'e unit cell. The combination of flat dispersion and uniformity of Berry curvature near the magic angle holds promise for realizing fractional quantum anomalous Hall effect at fractional filling. We also identify twist angles favorable for quantum spin Hall insulators and interaction-induced quantum anomalous Hall insulators at other integer fillings.