Kinky plasmons in double layers of borophene-borophene and borophene-graphene

TL;DR

This paper studies collective plasmon modes in double layers of 2D materials with tilted Dirac cones, revealing kink features inherited from monolayers and how different parameters influence plasmon dispersion.

Contribution

It provides a hydrodynamic analysis of plasmons in double-layer systems with tilted Dirac cones, highlighting the inheritance of plasmonic kinks and mode behaviors.

Findings

Double layers exhibit two plasmon branches: in-phase (~√q) and out-of-phase (~q).

Tilted Dirac cones induce kink features in plasmon dispersion inherited by both modes.

Different Fermi velocities and chemical potentials lead to multiple energy scales and kinks in plasmon modes.

Abstract

We investigate the collective plasmon modes in double layer of two dimensional materials where either one or both of the layers have tilted Dirac cone. Consistent with quite generic hydrodynamic treatment, similar to double layer graphene systems we find two branches of plasmons. The in-phase oscillations of the two layers disperses as $\sqrt q$, while the out-of-phase mode disperses as $q$. When even one of the layers hosts tilted Dirac cone spectrum, the plasmonic kink which is a salient feature of a monolayer of tilted Dirac cone is inherited by both of these branches. In double layers composed of graphene (nontilted) and borophene (tilted) where the two layers have two different Fermi velocities, the velocity scale of plasmonic modes is set by the greater of the two. The kink always takes place when each plasmon mode crosses an energy scale $\omega_{\rm kink}$. When the two layers have different chemical potentials, there will be two such scales, and each of in-phase and out-of-phase modes develop two kinks. Moreover we find that an additional linearly dispersing overdamped mode of monolayer tilted Dirac cone system survives in the double layer system.