Ballistic-phonon pulses in a solid-liquid He-II interface

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Abstract
Results are presented on the transmission of ballistic longitudinal (L) and transverse (T) phonon pulses from sapphire into liquid He-II at a temperature T<or=0.25K. Fast response time thin film transducers are used and broad-band (Planck type) phonon radiation is employed. The conversion of L- and T-mode pulses at the interface into He-II is resolved using time-of-flight detection, with a detector located at some distance from the surface in bulk He-II. The ballistic part of the transmitted energy flux is measured as a function of generator power and hydrostatic He-II pressure. The transmission probability for T-mode phonons is found to be higher than for the L-mode. The data also indicate a strong frequency down conversion at the interface, larger for the L-mode than for the T-mode.