High-frequency phonon transmission between a solid and superfluid 4He

The frequency dependence of phonon transmission through a solid-liquid 4He interface is studied on the basis of a simple model of the solid-liquid coupling. For single-quantum processes the transmission coefficient is calculated using a T-matrix formalism and shows extremely sharp peaks, where the transmission probability becomes unity, at frequencies near the roton minimum and photon maximum of the superfluid dispersion curve. Multi-excitation processes in the helium are also analysed but are found to be small. The conditions required for observation of transmission peaks are discussed with reference to recent experimental data on high-frequency phonon transmission and modifications are proposed. (author)