Ballistic-Phonon-Pulse Transmission through a Solid-Liquid He II Interface atT=0.25K

The transmission of ballistic longitudinal (L) and transverse (T) phonon pulses from sapphire to liquid He II at temperatures of $T=0.25\mathrm{}$ K has been measured as a function of the pulse power and He II hydrostatic pressure for the first time. Under these experimental conditions, the converted L- and T-mode pulses propagate ballistically through He II. The results indicate that the transmission probability is different for the L and T modes and that the intensity ratio of converted L and T modes changes with the He II pressure.