Fluctuation subharmonic and multiharmonic phonon transmission and Kapitza conductance between crystals with very different vibrational spectra.

It is shown that nonlinear interaction of bulk acoustic phonons with fluctuation resonant vibrations of a two-dimensional transition layer at clean interface between two media (a rigid and a soft crystal) with very different elastic properties leads to the subharmonic transmission across the interface of acoustic phonons incident from a rigid crystal with frequencies above the highest frequency in a soft crystal, and to the second- (and multi) harmonic transmission of acoustic phonons incident from a soft crystal. These inelastic interface dynamical phenomena can substantially contribute to the Kapitza thermal boundary conductance between solids with very different vibrational spectra (such as diamond and soft crystal asPb orAu) and to the thermal conductance across a helium-solid interface also.