Monopole and multipole plasmons in a two-dimensional system

TL;DR

This study uses high-resolution electron energy loss spectroscopy on monolayer graphene to reveal the existence of both monopole and multipole plasmons, with the latter being a novel observation in 2D electron systems.

Contribution

The paper demonstrates the experimental observation of a previously unreported multipole plasmon in a 2D electron gas, expanding understanding of plasmonic excitations in graphene.

Findings

Identification of monopole and multipole plasmon dispersions

Multipole plasmon observed for the first time in 2D systems

Dispersion ratios vary between 1.4 and 2 across substrates

Abstract

Using monolayer graphene as a model system for a purely two-dimensional (2D) electron gas, we show by energy electron loss spectroscopy, highly resolved both in energy and momentum, that there is a significant probability for the excitation of not only one but two dispersing losses. The appearance of both losses is independent of the substrate (we tested graphene on the Si face of 6H-SiC(0001), and on Ir(111) without and with intercalated Na layer), and the ratio of the slope in the dispersion curves varies between 1.4 (SiC) and 2. While the lower dispersion curve can be attributed to the excitation of the monopole plasmon, in agreement with theoretical model calculations, the upper dispersion branch has not been identified before for plasmonic excitations in a 2D electron gas, and we assign it to the excitation of a multipole sheet plasmon.