Changing character of electronic transitions in graphene: From single particle excitations to plasmons

TL;DR

This paper investigates the evolution of electronic excitations in graphene, demonstrating the transition from single particle states to plasmons, and compares theoretical predictions with experimental electron energy loss spectroscopy data.

Contribution

It provides a detailed analysis of the transition from single particle to collective excitations in graphene and confirms the existence of $ ext{pi}$ and $ ext{pi}+ ext{sigma}$ plasmons within specific wavevector ranges.

Findings

Existence of $ ext{pi}$ and $ ext{pi}+ ext{sigma}$ plasmons in graphene.

Transition from single particle to collective excitations.

Agreement between theoretical results and EELS measurements.

Abstract

In this paper we clarify the nature of $\pi$ and $\pi+\sigma$ electron excitations in pristine graphene. We clearly demonstrate the continuous transition from single particle to collective character of such excitations and how screening modifies their dispersion relations. We prove that $\pi$ and $\pi+\sigma$ plasmons do exist in graphene, though occurring only for a particular range of wavevectors and with finite damping rate. The particular attention is paid to compare the theoretical results with available EELS measurements in optical ($\mathrm{Q\approx 0}$) and other ($\mathrm{Q\neq 0}$) limits. The conclusions, based on microscopic numerical results, are confirmed in an approximate analytical approach.