Universal self-amplification channel for surface plasma waves

We present a theory of surface plasma waves (SPWs) in metals with arbitrary electronic collision rate τ-1. We show that there exists a universal intrinsic amplification channel for these waves, subsequent to the interplay between ballistic motions and the metal surface. We evaluate the corresponding amplification rate γ0, which turns out to be a sizable fraction of the SPW frequency ωs. We also find that the value of ωs depends on surface scattering properties, in contrast with the conventional theory.