Band structure effects on the Be(0001) acoustic-surface-plasmon energy   dispersion
  We report first-principles calculations of acoustic surface plasmons on the (0001) surface of Be, as obtained in the random-phase approximation of many-body theory. The energy dispersion of these collective excitations has been obtained along two symmetry directions. Our results show a considerable anisotropy of acoustic surface plasmons, and underline the capability of experimental measurements of these plasmons to {\it map} the electron-hole excitation spectrum of the quasi two-dimensional Shockley surface state band that is present on the Be(0001) surface. 
V. M. Silkin, J. M. Pitarke, E. V. Chulkov, B. Diaconescu, K. Pohl, L.   Vattuone, L. Savio, Ph. Hoffmann, D. Farias, M. Roc...