Probing the bulk plasmon continuum of layered materials through electron energy loss spectroscopy in a reflection geometry

TL;DR

This paper demonstrates that electron energy loss spectroscopy (EELS) in reflection geometry probes the bulk plasmon continuum in layered materials, revealing a direct spectral weight correspondence and offering a new perspective on surface and bulk plasmon interactions.

Contribution

It establishes a one-to-one mapping between the bulk plasmon continuum and the spectral weight in the surface loss function measured by HREELS, clarifying the connection between surface and bulk plasmon responses.

Findings

Surface loss function reflects bulk plasmon continuum in layered materials.

HREELS can probe long-wavelength bulk plasmon excitations.

Spectral weight in surface loss function corresponds to bulk plasmon response.

Abstract

A periodic arrangement of 2D conducting planes is known to host a (bulk) plasmon dispersion that interpolates between the typical, gapped behavior of 3D metals and a gapless, acoustic regime as a function of the out-of-plane wavevector. The semi-infinite system -- the configuration relevant to Electron Energy Loss Spectroscopy (EELS) in a reflection geometry, as in High Resolution EELS (HREELS) -- is known to host a surface plasmon that ceases to propagate below a cutoff wavevector. As the f-sum rule requires a finite response whether or not there exist sharp excitations, we demonstrate that what remains in the surface loss function -- the material response probed by HREELS -- is the contribution from the (bulk) plasmon of the infinite system. In particular, we provide a one-to-one mapping between the plasmon continuum and the spectral weight in the surface loss function. In light of this result, we suggest that HREELS be considered a long wavelength probe of the plasmon continuum in layered materials.