Substrate polarization effects on the plasmon excitations of small Na atomic chains on Si surfaces

Abstract

The plasmon excitations in small metal-atom clusters have been extensively investigated in recent years, and most of the efforts are focused on freestanding clusters. However, freestanding clusters are inherently unstable and have to be stabilized by interacting with a two-dimensional or three-dimensional substrate; hence, the effects of substrates should be taken into account in the calculations of electronic excitations. With the use of the random-phase approximation, we extensively investigate the polarization effects of substrates on the plasmon resonances of small Na chains on Si surfaces. Our results show that due to the polarization effects of Si substrates, the redshifts of plasmon peaks appear in both longitudinal and transverse absorption spectra, and the strengths of the main plasmon resonant peaks have some variation; in particular, the strength gets a very large enhancement when the external field is applied in the direction perpendicular to the surface of Si. In addition, we study the formation mechanism of the end mode and the central mode of transverse-plasmon excitations and show that the two modes are caused by electron-electron interactions, which reflects the collective excitations induced by the interactions.