Collective excitations in two-component one-dimensional massless Dirac plasma

TL;DR

This paper investigates the spectra of long wavelength plasma oscillations in a two-component 1D massless Dirac fermion system with spin-orbit coupling, revealing quantum and classical plasmon branches and their interactions.

Contribution

It introduces a detailed analysis of intrasubband and intersubband plasmons in 1D Dirac systems with spin-orbit interaction, highlighting new quantum and classical behaviors.

Findings

Optical intrasubband plasmon is quantum with inverse square root dependence on Planck's constant.

Acoustic intrasubband plasmon is classical with velocity related to edge subband velocities.

Interband plasmons exhibit gapped branches with positive and negative group velocities, affected by energy splitting.

Abstract

We study spectra of long wavelength plasma oscillations in a system of two energy splitted one-dimensional (1D) massless Dirac fermion subbands coupled by spin-orbit interaction. Such a system may be formed by edge subbands in semiconducting transition metal dichalcogenide monolayers. Intrasubband transitions of massless Dirac fermions give rise to optical and acoustic gapless branches of intrasubband 1D plasmons. We reveal that the optical branch is of quantum character with group velocity being inverse proportional to square root of the Planck constant, whereas the acoustic branch is classical one with group velocity proportional to geometric mean of the edge subband velocities. Spin-orbit interaction, allowing intersubband transitions in the system, results in emergence of two branches of intersubband 1D plasmons: upper and lower ones. The upper and lower branches are gapped at small wave vectors and evolve with positive and negative group velocities, respectively, from energy splitting of the edge subbands at Fermi-level. The both intersubband branches adjoin intersubband single particle excitation continuum from above, while in case of the edge subbands with unequal velocities the lower one experiences Landau damping at small wave vectors. In addition, the lower branch, attaining zero frequency at a non-zero wave vector, alters its group velocity from negative to positive one.