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2D metallic systems formed in charge inversion layers and artificial layered materials permit the existence of low-energy collective excitations, so-called 2D plasmons, which are not found in a 3D metal. These excitations have caused considerable interest because their low energy allows them to participate in many dynamical processes involving electrons and phonons and as possible candidates to mediate the attractive interaction responsible for the formation of Cooper pairs in high Tc superconductors. Metals often support electronic states that are confined to the surface forming a nearly 2D electron density layer and they were recently predicted to support sound like plasmon excitations [1] that could also affect electron-hole and phonon dynamics near the Fermi level. Until then, the accepted understanding was that metallic surfaces should only support the conventional surface plasmons, modes with energies of a few eV depending only on the electron density, with important applications in surface-plasmon resonance microscopy, photonics and sub-wavelength optics, but no relevance to the low-energy dynamics. We show that, in contrast to this well-established belief, a low-energy collective excitation mode can be found on bare metal surfaces. The mode has an acoustic (linear) dispersion, different to the q1/2 dispersion of a 2D plasmon, and was observed on Be(0001) using angle-resolved electron energy loss spectroscopy. First-principles calculations show that it is caused by the coexistence of a partially occupied quasi 2D surface-state band with the underlying 3D bulk electron continuum and that the non-local character of the dielectric function prevents it from being screened out by the 3D states. Its dispersion is mainly determined by the surface-state Fermi velocity, v2D, and follows closely the upper edge of the continuum for electron-hole pair excitations within the surface-state band. While for Be(0001) a high value of v2D warrants the plasmon's excitation at higher than 1 eV, thus facilitating its observation, the mode as such has a very general character and should be present on many metal surfaces profoundly affecting their electron and phonon dynamics.
[1] V. M. Silkin et. al., Phys. Rev. B 72, 115435 (2005) |