Phonon detection by the fountain pressure in superfluid 4Helium films

The importance of phonon focussing was first demonstrated by Taylor, Maris and Elbaum, who calculated the phonon intensities or different modes in single crystals from the detailed form of the angle or k-dependent sound velocity surfaces. The results of these calculations are well-confirmed by experiment. It is quite surprising that phonon focussing leads in several crystals to very narrow beams of phonon energy propagation especially for transverse modes, as recently calculated by Rosch and Weis, who presented their computer results in a very instructive form. From these theoretical results it appears worthwhile to devise methods by which the intensity distribution of incoherent phonons propagating in single crystals can be measured in detail or directly imaged