Novel low-energy collective excitation at metal surfaces
  A novel collective excitation is predicted to exist at metal surfaces where a two-dimensional surface-state band coexists with the underlying three-dimensional continuum. This is a low-energy acoustic plasmon with linear dispersion at small wave vectors. Since new modern spectroscopies are especially sensitive to surface dynamics near the Fermi level, the existence of surface-state induced acoustic plasmons is expected to play a key role in a large variety of new phenomena and to create situations with potentially new physics. 
V. M. Silkin, A. Garcia-Lekue, J. M. Pitarke, E. V. Chulkov, E.   Zaremba, and P. M. Echenique