Sound propagation in a superleak covered with solid 4He

2014 Effects of crystal growth on the fourth sound propagation are studied in the framework of hydrodynamics. The particular system we have in mind is superfluid 4He in a grafoil superleak. Solid 4He is formed on the surface of graphite and it grows continuously when the pressure of liquid is increased during passage of a sound wave. Sound velocity and Doppler shifts for steady superfluid flow are calculated as a function of the strength of the substrate potential (or equivalently the thickness of the solid layer) and the rate of crystal growth. In the limit of strong substrate potential (thin solid layer) or slow crystal growth they are the same as the usual fourth sound. On the other hand, if the potential is weak and the crystal growth is fast, we find several new features : the sound velocity approaches zero at T = 0 because the fast crystal growth does not allow pressure changes of the liquid, but at finite temperatures the change of entropy density on crystallization (or melting) assures propagation of the sound The Doppler shift is much larger than the velocity of the flow and is in the opposite sense. Experimental conditions for grafoil superleaks are considered. J. Physique 45 (1984) 1559-1569 SEPTEMBRE 1984, 1 Classification Physics Abstracts 67.80 67.40 81.1OF