Correlated insulators, density wave states, and their nonlinear optical response in magic-angle twisted bilayer graphene

TL;DR

This paper theoretically investigates the correlated insulator and density wave states in magic-angle twisted bilayer graphene, proposing nonlinear optical response as a novel experimental method to identify and distinguish these elusive states.

Contribution

It introduces a theoretical framework including Coulomb screening effects to analyze the nonlinear optical response of correlated states in TBG, suggesting a new experimental probe.

Findings

Nonlinear optical conductivities vary with different correlated states.

Nonlinear optical response can distinguish between insulator and density wave states.

Proposes nonlinear optics as a practical tool for characterizing TBG states.

Abstract

The correlated insulator (CI) states and the recently discovered density wave (DW) states in magic-angle twisted bilayer graphene (TBG) have stimulated intense research interest. However, up to date, the nature of these "featureless" correlated states with zero Chern numbers are still elusive, and are lack of characteristic experimental signature. Thus, an experimental probe to identify the characters of these featureless CI and DW states are urgently needed. In this work, we theoretically study the correlated insulators and density-wave states at different integer and fractional fillings of the flat bands in magic-angle TBG based on extended unrestricted Hartree-Fock calculations including the Coulomb screening effects from the remote bands. We further investigate the nonlinear optical response of the various correlated states, and find that the nonlinear optical conductivities can be used to identify the nature of these CI and DW states at most of the fillings. Therefore, we propose that nonlinear optical response can serve as a promising experimental probe to unveil the nature of the CI and DW states observed in magic-angle TBG.