ECOSS 25 - European Conference on Surface Science, Liverpool, UK, 7/28 - 7/31/07

Acoustic surface plasmons: A novel collective excitation on metal surfaces

Karsten Pohl

Department of Physics and Materials Science Program, University of New Hampshire, Durham, NH, U.S.A.

The recent discovery of a fundamentally new sound-like plasmon on a bare metal surface of beryllium may introduce a new research direction in the area of plasmonics [1]. While conventional surface plasmons are optical modes and have a finite excitation energy of a few eV, the novel acoustic mode can be excited with very low energies of a few meV. This allows, in principle, for a coupling with visible light for signal processing and advanced microscopies as well as new catalysts on metallic surfaces. In order to show that this novel excitation is a general phenomenon on closed-packed noble metal surfaces, as predicted by our theoretical collaborators [2], we have measured the dispersion of the acoustic surface plasmon on Cu(111) by electron energy-loss spectroscopyfor a parallel momentum-transfer range from 0 to 0.20 1/Å. We can report that the dispersion is indeed linear (acoustic) with a slope (sound velocity) in good agreement with theory [2] and energy values that extend up to 1 eV. We will shown that the novel acoustic surface plasmon, ASP, is a general phenomenon on metal surfaces that support a partially occupied surface state within a wide bulk energy gap. It is caused by the non-local screening of the surface electrons due to bulk electron. The adsorption of about 0.25 ML of oxygen on the surface removes the surface state on Cu(111) and indeed destroys the ASP.

[1] B. Diaconescu, K.Pohl, L. Vattuone, et al., Nature 448, 57 (2007)

[2] V.M. Silkin, J.M. Pitarke, et al., Phys. Rev. B 72, 115435 (2005)