Correlated states in magic angle twisted bilayer graphene under the optical conductivity scrutiny

TL;DR

This paper demonstrates how optical conductivity measurements can identify different correlated and symmetry-breaking states in magic angle twisted bilayer graphene, revealing details about nematic order and lattice effects.

Contribution

It provides a theoretical framework for distinguishing symmetry-breaking states in TBG using optical conductivity, especially highlighting the effects of nematic order.

Findings

Nematic order displaces Dirac cones in momentum and energy.

Finite Drude weight appears at the charge neutrality point due to nematic order.

Conductivity anisotropy depends on lattice relaxation, doping, and symmetry breaking.

Abstract

Moir\'e systems displaying flat bands have emerged as novel platforms to study correlated electron phenomena. Insulating and superconducting states appear upon doping magic angle twisted bilayer graphene (TBG), and there is evidence of correlation induced effects at the charge neutrality point (CNP) which could originate from spontaneous symmetry breaking. Our theoretical calculations show how optical conductivity measurements can distinguish different symmetry breaking states, and reveal the nature of the correlated states. In the specific case of nematic order, which breaks the discrete rotational symmetry of the lattice, we find that the Dirac cones are displaced, not only in momentum space but also in energy, inducing finite Drude weight at the CNP. We also show that the sign of the dc conductivity anisotropy induced by a nematic order depends on the degree of lattice relaxation, the doping and the nature of the symmetry breaking.