Critical role of device geometry for the phase diagram of twisted bilayer graphene

TL;DR

This study investigates how device geometry, especially gate separation, influences the electronic phase transitions in twisted bilayer graphene by analyzing the Hubbard parameters and screening effects.

Contribution

It introduces an atomistic model to quantify how gate separation affects electron correlations and phase transitions in tBLG, considering various device configurations.

Findings

Critical gate separation determines transition from insulator to metal.

Screening effects depend on device geometry, twist angle, and doping.

Correlated insulator states can be suppressed in devices with thin dielectric layers.

Abstract

The effective interaction between electrons in two-dimensional materials can be modified by their environment, enabling control of electronic correlations and phases. Here, we study the dependence of electronic correlations in twisted bilayer graphene (tBLG) on the separation to the metallic gate(s) in two device configurations. Using an atomistic tight-binding model, we determine the Hubbard parameters of the flat bands as a function of gate separation, taking into account the screening from the metallic gate(s), the dielectric spacer layers and the tBLG itself. We determine the critical gate separation at which the Hubbard parameters become smaller than the critical value required for a transition from a correlated insulator state to a (semi-)metallic phase. We show how this critical gate separation depends on twist angle, doping and the device configuration. These calculations may help rationalise the reported differences between recent measurements of tBLG's phase diagram and suggests that correlated insulator states can be screened out in devices with thin dielectric layers.