Theory of anisotropic plasmons

We develop the complete theory for collective plasmon modes of an interacting electron system in presence explicit mass (or velocity) anisotropy corresponding noninteracting situation, with effective Fermi velocity being different along axes. Such is common solid state materials (e.g., silicon or germanium), where surface often not spherical. find that dispersion itself develops significant such systems, and commonly used isotropic approximation using a density states optical does work anisotropic system. predict qualitatively new phenomenon systems no analog, mode one direction decays into electron-hole pairs through Landau damping while remains undamped stable direction.