Dynamical properties of collective excitations in twisted bilayer Graphene

TL;DR

This study uses advanced quantum Monte Carlo simulations to explore the dynamic excitations in twisted bilayer graphene, revealing competing symmetry-breaking states and valley wave behaviors that resemble magnetic spin waves.

Contribution

It introduces a momentum-space quantum Monte Carlo approach to analyze collective excitations in realistic models of twisted bilayer graphene, uncovering new insights into symmetry breaking and valley dynamics.

Findings

Strong competition between symmetry-breaking channels at charge neutrality.

Repulsive interactions induce an insulating gap in the spectrum.

Long-lived valley waves resemble Heisenberg ferromagnetic spin waves.

Abstract

Employing the recently developed momentum-space quantum Monte Carlo scheme, we study the dynamic response of single-particle and collective excitations in realistic continuum models of twisted bilayer graphene. At charge neutrality, this unbiased numerical method reveals strong competition between different symmetry breaking channels with a leading instability towards the intervalley coherent state. Single-particle spectra indicate that repulsive interactions push the fermion spectral weight away from the Fermi energy and open up an insulating gap. The spectra of collective excitations suggest an approximate valley $SU(2)$ symmetry. At low-energy, long-lived valley waves are observed, which resemble spin waves of Heisenberg ferromagnetism. At high-energy, these sharp modes quickly become over-damped, when their energy reaches the fermion particle-hole continuum.