A Universal Self-Amplification Channel for Surface Plasma Waves

We present a theory of surface plasma waves (SPWs) in metals with arbitrary electronic collision rate $1/\tau$. We show that there exists a universal intrinsic amplification channel for these waves, as a result of the unique interplay between ballistic electronic motions and the metal surface. The corresponding intrinsic amplification rate $\gamma_0$ is shown to be independent of $\tau$. We also study its dependence on surface scattering properties.