Band structure versus dynamical exchange-correlation effects in surface plasmon energy and damping: a first-principles calculation

Evidence for a spin acoustic surface plasmon from inelastic atom scattering

Closed-shell atoms scattered from a metal surface exchange energy and momentum with surface phonons mostly via the interposed surface valence electrons, i.e., via the creation of virtual electron-hole pairs. The latter can then decay into surface phonons via electron-phonon interaction, as well as into acoustic surface plasmons (ASPs). While the first channel is the basis of the current inelastic atom scattering (IAS) surface-phonon spectroscopy, no attempt to observe ASPs with IAS has been made so far. In this study we provide evidence of ASP in Ni(111) with both Ne atom scattering and He atom scattering. While the former measurements confirm and extend so far unexplained data, the latter illustrate the coupling of ASP with phonons inside the surface-projected phonon continuum, leading to a substantial reduction of the ASP velocity and possibly to avoided crossing with the optical surface phonon branches. The analysis is substantiated by a self-consistent calculation of the surface response function to atom collisions and of the first-principle surface-phonon dynamics of Ni(111). It is shown that in Ni(111) ASP originate from the majority-spin Shockley surface state and are therefore collective oscillation of surface electrons with the same spin, i.e. it represents a new kind of collective quasiparticle: a Spin Acoustic Surface Plasmon (SASP).

Correlated motion of electrons on the Au(111) surface: anomalous acoustic surface-plasmon dispersion and single-particle excitations

The linear dispersion of the low-dimensional acoustic surface plasmon (ASP) opens perspectives in energy conversion, transport, and confinement far below optical frequencies. Although the ASP exists in a wide class of materials, ranging from metal surfaces and ultrathin films to graphene and topological insulators, its properties are still largely unexplored. Taking Au(111) as a model system, our combined experimental and theoretical study revealed an intriguing interplay between collective and single particle excitations, causing the ASP associated with the Shockley surface state to be embedded within the intraband transitions without losing its sharp character and linear dispersion.

Validity of the semi-classical approach for calculation of the surface excitation parameter

The problem of surface plasmon excitation by moving charges has been elaborated by several different approaches, mainly based on dielectric response theory within either semi-classical or quantum mechanical frameworks. In this work, a comparison of the surface excitation effect between two different frameworks is made by calculation of the differential inverse inelastic mean free path (DIIMFP) and a Monte Carlo simulation of reflection electron energy loss spectroscopy (REELS) spectra. A semi-classical modeling of the interaction between electrons and a solid surface is based on analyzing the work done by moving electrons; the stopping power and inelastic cross section are derived with the induced potential. On the other hand, a quantum mechanical approach is based on derivation of the complex inhomogeneous self-energy of the electrons. The numerical calculation shows that the semi-classical model presents almost the same values of DIIMFP as by the quantum model except at the glancing condition. The simulation of REELS spectra for Ag and SiO(2) as well as a comparison with experimental spectra also confirms that a good agreement with the spectral shape is found among the two simulation results and the experimental data.

Nonlocal screening of plasmons in graphene by semiconducting and metallic substrates: first-principles calculations

We investigate the role of substrates on the collective excitations of graphene by using a first-principles implementation of the density response function within the random-phase approximation. Specifically, we consider graphene adsorbed on SiC(0001) and Al(111) as representative examples of a semiconducting and metallic substrate. On SiC(0001), the long wavelength π plasmons are significantly damped although their energies remain almost unaltered. On Al(111), the long wavelength π plasmons are completely quenched due to the coupling to the metal surface plasmon. The strong damping of the plasmon excitations occurs despite the fact that the single-particle band structure of graphene is completely unaffected by the substrates illustrating the nonlocal nature of the effect.

Plasmon dispersion of the Au(111) surface with and without self-assembled monolayers

The surface plasmon dispersion of gold films with and without chemisorbed, alkane thiol self-assembled monolayers (SAMs) has been investigated using high resolution electron energy loss spectroscopy (HREELS). For a bare Au(111) film, the surface plasmon energy (2.49 eV at the zone center) shows a positive dispersion. After adsorption of ethylbenzenethiol or dodecanethiol SAMs, the plasmon energy at the zone center blueshifts and the dispersion switches sign to become negative, thus mimicking the behavior of a free-electron system. This striking behavior represents a benchmark for models of the electronic structure of the gold-sulfur interface, as manifest both in SAMs and in monolayer-protected nanoparticles.

Surface plasmon and electron-hole structures in the excitation spectra of thin films

Surface excitation spectra are calculated, including both collective and single-particle modes, and examined in detail. This is achieved by calculating the non-local dielectric function ε(p)(Q,z,z('),ω) of the thin jellium film within the random phase approximation (RPA) (using local density approximation wavefunctions which actually takes us beyond the RPA), from which we then derive the spectral function. The high precision of the calculations enables us to analyse not only the collective (surface plasmon) modes and their dependence on the film thickness, but also the intra-band electron-hole excitations, and for the first time oscillatory structures due to inter-band transitions. The spectra are then analysed with special attention to their dependence on the slab thickness, and the periodic peaks observed due to single-particle excitations in the spectra.