HREELS Study on Plasmon Excitations of Low-dimensional Systems

Abstract

Plasmons, which are collective excitations of charge carriers, have been extensively studied on account of their importance in condensed matter physics and their potential application in the fabrication of enhanced, highly efficientnanoscale devices. With the advance in technology, plasmons could eventually be confined to a small volume of atomic-scale, and they are expected to have unusual physical properties because of their low-dimensional electronic systems.Here, we report our experimental results for electronic excitations measuredfrom two prototypical atomic-scale systems using high-resolution electron energy loss spectroscopy.As a rst system, we have studied several Au-induced facet structures formedon Si(5512) surface, where several quasi-one dimensional plasmon excitationswith signifficantly anisotropic energy-momentum dispersion have been observed. Those dispersions of plasmon excitations appear to be quite similar to each other despite signifficant dierences in the atomic structures of theirsubstrate surfaces. We suggest the spin-orbit interaction as an origin of suchsimilarity in the dispersions but also of the two proximal surface bands nearFermi level (EF ) commonly observed for these Au-induced 1D systems. We also find the interchain interactions between metallic chains minimal.We have also investigated the dispersion of a low-energy intraband π-plasmon arising from Dirac fermions within a partially occupied conductionband of single-layer graphene (SLG). The dispersion from the SLG appears tobe quite distinct from that of normal two dimensional systems or of a few-layergraphene. Discussion of the physical implications of the novel dispersion fromthe SLG refers to a recently developed theory on doped graphene.Finally, we report that the behavior of plasmon excitation in SLG can bemodified by doping external potassium (K) atoms without altering the uniqueplasmonic character of the SLG. This capability should provide a good opportunity in developing noble graphene-based nano-devices with enhanced performance.