  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.
