Acoustic surface plasmons in the noble metals Cu, Ag, and Au
  We have performed self-consistent calculations of the dynamical response of the (111) surface of the noble metals Cu, Ag, and Au. Our results indicate that the partially occupied surface-state band in these materials yields the existence of acoustic surface plasmons with linear dispersion at small wave vectors. Here we demonstrate that the sound velocity of these low-energy collective excitations, which had already been predicted to exist in the case of Be(0001), is dictated not only by the Fermi velocity of the two-dimensional surface-state band but also by the nature of the decay and penetration of the surface-state orbitals into the solid. Our linewidth calculations indicate that acoustic surface plasmons should be well defined in the energy range from zero to $\sim 400$ meV. 
V. M. Silkin, J. M. Pitarke, E. V. Chulkov, and P. M. Echenique