Excitonic Metal and Non-Fermi Liquid Behaviour in Twisted Double Bilayer Graphene near Charge Neutrality

TL;DR

This paper investigates how strong Coulomb interactions in twisted double bilayer graphene near charge neutrality lead to exciton formation, resulting in an excitonic metal with charge density wave order and non-Fermi liquid behavior affecting low-temperature transport.

Contribution

It introduces the concept of an excitonic metal in twisted double bilayer graphene and explains its impact on transport properties and non-Fermi liquid behavior near charge neutrality.

Findings

Formation of indirect excitons due to Coulomb attraction.

Emergence of an excitonic metal with charge density wave order.

Observation of sublinear T^{2/3} resistivity near charge neutrality.

Abstract

Twisted double bilayer graphene is a compensated semi-metal near the charge neutrality point with the presence of small electron and hole pockets in its band structure. We show that strong Coulomb attraction between the electrons and holes can lead to the formation of indirect excitons. Condensation of these excitons at low temperature creates an excitonic metal with charge density wave order for an appropriate range of interaction strength. This has interesting implications for low-temperature transport in the system as a function of carrier density and temperature. The reorganization of the single particle excitations and their density of states in the excitonic metal can lead to peaks in resistivity as a function of carrier density, recently seen in experiments at low temperatures. The fluctuations of the Landau damped order parameter in the quantum critical metal lead to non-Fermi liquid behaviour, which can explain the sublinear $T^{2/3}$ dependence of the resistance near the charge neutrality point.