Theory of acoustic surface plasmons

TL;DR

This paper models the existence of acoustic surface plasmons at metal surfaces with coexisting 2D surface states and 3D bulk states, showing they have linear dispersion due to nonlocal screening effects.

Contribution

It introduces a self-consistent model incorporating 3D screening into 2D electron gas excitations, demonstrating the universal existence of acoustic surface plasmons.

Findings

Acoustic surface plasmons exist for all 2D sheet positions relative to the metal surface.

These plasmons exhibit linear dispersion at low wave vectors.

Nonlocal 3D response causes incomplete screening, enabling these excitations.

Abstract

Recently, a novel low-energy collective excitation has been predicted to exist at metal surfaces where a quasi two-dimensional (2D) surface-state band coexists with the underlying three-dimensional (3D) continuum. Here we present a model in which the screening of a semiinfinite 3D metal is incorporated into the description of electronic excitations in a 2D electron gas through the introduction of an effective 2D dielectric function. Our self-consistent calculations of the dynamical response of the 3D substrate indicate that an acoustic surface plasmon exists for all possible locations of the 2D sheet relative to the metal surface. This low-energy excitation, which exhibits linear dispersion at low wave vectors, is dictated by the nonlocality of the 3D dynamical response providing incomplete screening of the 2D electron-density oscillations.