Phonon Scattering at Surfaces and Interfaces

Since the discovery of the thermal boundary resistance by Kapitza /1/ in 1941, there have been extensive experimental and theoretical studies on the underlying phonon transmission, reflection and scattering processes at surfaces and interfaces. The field has been recently reviewed by A.C. Anderson /2/ and A.F.G. Wyatt /3/. The reader is referred to these authors, since the present contribution cannot give a complete account of all the relevant work done. For ideal solid-liquid or solid-solid boundary conditions the theoretical treatment according to the acoustic mismatch model by Khalatnikov /4/ and Little /5/ predicts strong phonon reflection at the solid-liquid 4He interface, but weak phonon reflection for most solid-metal interfaces. In the latter case the acoustic model was especially successful for the calculation of phonon spectra and intensity distributions emitted by metallic heaters into dielectric substrates as performed by Weis and co-workers /6/ taking also account of phonon focussing /7/. Experimental evidence for the applicability of the acoustic model to the solid-liquid 4He interface for phonon frequencies ranging from 100 to about 700 GHz was provided by the famous UHV cleaving experiments of Kinder and co-workers /8/. More recently Basso et al. /9/ demonstrated that phonons of 100 6Hz are almost ideally reflected from Si-liquid 4He interfaces after laser annealing the Si-crystal under UHV conditions.