Plasmon resonances in linear atomic chains: Free-electron behavior and anisotropic screening ofdelectrons

Abstract

Electronic excitations in linear atomic chains of simple and noble metals (silver) have been studied using time-dependent density-functional theory. The formation and development of collective resonances in the absorption spectra were obtained as functions of the chain length. A longitudinal collective resonance appears in both simple- and noble-metal chains. Its dispersion has been deduced and is compared with that of a one-dimensional electron gas. The transverse excitation generally shows a bimodal structure, which can be assigned as the ``end and central resonances.'' The $d$ electrons of silver atoms reduce both the energies and intensities of the transverse modes but have little effect on its longitudinal resonance. This anisotropic screening is determined by the interband $(d\ensuremath{\rightarrow}p)$ transition, which is involved only in transverse oscillations. Analysis of these results yields a general picture of plasmon resonances in one-dimensional atomic structures. Implications of such atomic-scale plasmons to surface plasmons in larger dimensions are also discussed.