Plasmonic Quantum Dots in Twisted Bilayer Graphene

TL;DR

This paper develops a realistic many-body Hamiltonian for twisted bilayer graphene, incorporating detailed screening effects, and predicts twist-dependent plasmonic quantum-dot-like excitations with distinct symmetries and optical properties.

Contribution

It introduces a comprehensive first-principles-based model for low-energy electronic interactions and plasmonic excitations in twisted bilayer graphene, including non-local screening effects from high-energy states.

Findings

Identification of twist-angle-dependent plasmonic quantum-dot-like modes.

Classification of modes into bright and dark states based on charge modulations.

Prediction of distinct optical signatures in electron energy loss spectra.

Abstract

We derive a material-realistic real-space many-body Hamiltonian for twisted bilayer graphene from first principles, including both single-particle hopping terms for $p_z$ electrons and long-range Coulomb interactions. By disentangling low- and high-energy subspaces of the electronic dispersion, we are able to utilize state-of-the-art constrained Random Phase Approximation calculations to reliably describe the non-local background screening from the high-energy $s$, $p_x$, and $p_y$ electron states for arbitrary twist angles. The twist-dependent low-energy screening from $p_z$ states is subsequently added to obtain a full screening model. We use this approach to study real-space plasmonic patterns in electron-doped twisted bilayer graphene supercells and find, next to classical dipole-like modes, also twist-angle-dependent plasmonic quantum-dot-like excitations with $s$ and $p$ symmetries. Based on their inter-layer charge modulations and their footprints in the electron energy loss spectrum, we can classify these modes into "bright" and "dark" states, which show different dependencies on the twist angle.