ECOSS 24 - European Conference on Surface Science, Paris, France, 9/4 - 9/8/06

Acoustic Collective Excitation of a 2D Electron Gas on Metal Surfaces

B. Diaconescu, K. Pohl (UNH), L. Vattuone, L. Savio (Genova), Ph. Hofmann (Arhus), V. Silkin, E.Chulkov, P. Echenique (San Sebastian), D. Farias (Madrid), M. Rocca (Genova)

Sound-like longitudinal plasma waves where thought to only exist in layered systems, where spatially separated two-dimensional electron plasmas are realized. Due to their low excitation energy and linear dispersion such waves were proposed as possible candidates to mediate the attractive interaction leading to the formation of Cooper pairs in high TC superconductors.1 Many metal surfaces support filled surface states and they were recently predicted to support sound like plasmon excitations that could also affect electron-hole and phonon dynamics near the Fermi level.2 In this work we report on the experimental measurement and theoretical description of a collective acoustic excitation on a metal surface. The (0001) surface of beryllium supports a partially occupied electronic surface state band which leads to a strong increase in the local density of states at the surface close to the Fermi energy. Such strongly localized surface states can be considered to form a two-dimensional electron gas coexisting in the same region of space with the underlying three-dimensional continuum.

We have performed angle resolved electron energy loss spectroscopy (EELS) measurements on Be(0001) and were able, for the first time, to confirm the predicted existence of a new low-energy collective excitation with an acoustic linear dispersion at low values of parallel momentum transfer.2 Measurements were performed up to 2 eV loss energy corresponding to a parallel momentum transfer of 0.38Å-1. Such acoustic modes have so far escaped experimental observation because of the stringent kinematical conditions necessary for their excitation in EELS experiments. Our detailed first principle calculations show that a partially occupied quasi-2D surface state band coexisting in the same region of space with the underlying 3D continuum is responsible for such collective excitations.

1 N. H. March and M. P. Tosi, Adv. Phys. 44, 299 (1995)
2 V. M. Silkin et al., Phys. Rev. B 72, 115435 (2005)