Oscillating Dependence of the Acoustic Attenuation Coefficient in Thin Metal Layers

Abstract

Dimensional and frequency dependences of the energy absorption coefficient Γ of a longitudinal sound wave in a plane parallel metal layer are theoretically investigated for arbitrary relationship between the layer thickness d and the electron free path l. Exact and asymptotic (for thick (d >> l) and thin (d << l) metal layers) formulas are derived for the coefficient Γ for an arbitrary angular dependence of the probability of specular reflection of charge carriers from the sample surface q(ϕ). A square-root dependence of the acoustic absorption coefficient on the thin film thickness is predicted. If the direction of sound wave propagation is perpendicular to the thin metal layer boundary, the acoustic absorption coefficient becomes an oscillating function of the layer thickness; its amplitude decreases with increasing kd, where k is the acoustic wave number.