Phononlike Surface Modes in Two-Component Plasmas and in Systems with Short-Range Interactions

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Abstract
Surface modes in a homogeneous system with a sharp boundary are discussed using the collisionless Boltzmann equation for the Wigner distribution function. In a one-component system of fermions interacting via a Yukawa potential, we show that there is an acoustic surface mode. In the limit of long wavelengths, it becomes indistinguishable from the analogous bulk zero-sound mode. For a system of electrons and ions, we find a low-frequency in-phase surface mode, in addition to a high-frequency out-of-phase optical surface mode. For long wavelengths, the low-frequency mode is phononlike with a speed identical to that given by the Bohm-Staver expression.