Theory of plasmonic edge states in chiral bilayer systems

TL;DR

This paper analytically describes plasmonic edge modes in twisted bilayer graphene and similar heterostructures, revealing how chirality influences edge plasmon dispersion and enabling potential near-field nanoscopy applications.

Contribution

It introduces an analytical model for edge plasmon dispersion in chiral bilayer systems, highlighting the role of chirality as an effective magnetic field and predicting nearly undamped acoustic modes.

Findings

Edge mode dispersion depends on chiral response even in nonretarded limit.

Universal function for optical edge plasmon dispersion in paramagnetic regime.

Chirality acts as an effective magnetic field, enabling nearly undamped acoustic modes.

Abstract

We analytically describe the plasmonic edge modes for an interface that involves the twisted bilayer graphene (TBG) or other similar Moire van der Waals heterostructure. For this purpose, we employ a spatially homogeneous, isotropic and frequency-dependent tensor conductivity which in principle accounts for electronic and electrostatic interlayer couplings. We predict that the edge mode dispersion relation explicitly depends on the chiral response even in the nonretarded limit, in contrast to the collective bulk plasmonic excitations in the TBG. We obtain a universal function for the dispersion of the optical edge plasmon in the paramagnetic regime. This implies a correspondence of the chiral-TBG optical plasmon to a magnetoplasmon of a single sheet, and chirality is interpreted as an effective magnetic field. The chirality also opens up the possibility of nearly undamped acoustic modes in the paramagnetic regime. Our results may guide future near-field nanoscopy for van der Waals heterostructures. In our analysis, we retain the long-range electrostatic interaction, and apply the Wiener-Hopf method to a system of integral equations for the scalar potentials of the two layers.