REFLECTION OF HIGH-FREQUENCY PHONONS AT SILICON-SOLID INTERFACES

In reflection experiments with phonons of frequencies above 280 GHz propagating along (110) directions we observed large deviations from the acoustic mismatch theory for silicon-metal. silicon-condensed gas, and silicon-liquid helium in-terfaces. In quantitative phonon generation and detection ex-periments with superconducting tunneling junctions Trumpp measured excessive 2A-phonon losses of 90 to 98%, as compared with the predictions of the acoustic mismatch model [1-3]. He located the main loss source at the boundaries between the junctions and the substrate. This phonon loss considerably reduces the sensi-tivity in phonon spectroscopy experiments [4,5] As it is related to the well-known anomaly of the Kapitza resistance between solids and helium [6,7], this pho-non loss motivated the present reflection experiments