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Acta Physica Sinica

THE MODE OF SOUND IN GASEOUS SUPERFLUID FILM OF SPIN POLARIZED HYDROGEN ATOMS (H↓) ABSORBED ON HELIUM FILM

NI WAN-SUN, 南京大学物理系及声学研究所

Published Jan 1, 1983• 0 citations
Publisher

Abstract

The mode of sound in gaseous superfluid film of spin polarized hydrogen atoms (H↓) absorbed on helium film is discussed. Principally, it ia the fourth sound in two dimensions resulting from compressibility of the gaseous film and different from the usual third sound resulting from the variances of thickness of helium film.
DOI
10.7498/aps.32.1017

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Acta Physica Sinica•Jan 1, 1983•0 citations

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The probability of sticking of spin-polarised atomic hydrogen, H down arrow , on the superfluid helium surface as a function of temperature from 73 to 526 mK has been identified to arise from its interaction with third-sound ripplons. When the number density of the spin-polarised hydrogen atoms in the bulk helium becomes equal to that of the third-sound ripplons, the rate of sticking of H down arrow on the surface of the superfluid helium becomes a linear function of temperature. The predicted linearity is in accord with the experimental measurements.

Journal of Physics C Solid State Physics•Jul 10, 1987•0 citations
In order to minimize the possibility of spin flips due to interactions with and on the walls of a sample cell, spin-polarized hydrogen (H\ensuremath{\downarrow}) is studied in a 4He^{4}\mathrm{He}4He sample cell, i.e., a sample cell which has been coated with a film of superfluid 4He^{4}\mathrm{He}4He. At low temperatures it is expected that the most dense H\ensuremath{\downarrow} sample will first form in the physisorbed state on the 4He^{4}\mathrm{He}4He film. In this paper we examine in detail the hydrodynamic consequences of this layered film configuration. We derive the expression for third sound in the general case that the films are compressible and both films have undergone superfluid transitions. In this case two propagating modes will be seen which will provide an uneqivocal demonstration of a superfluid transition in the H\ensuremath{\downarrow} film. We specialize to various limiting cases: incompressible films, saturated and unsaturated films, and the normal fluid H\ensuremath{\downarrow} blanket. In our discussion we stress the point that even if the H\ensuremath{\downarrow} has not gone superfluid it will still shift the 4He^{4}\mathrm{He}4He third-sound speed, that this shift is a direct measure of the H\ensuremath{\downarrow} film areal density, and that the magnitude of this shift is expected to be well within the precision of present-day third-sound experimental techniques. Thus third sound is expected to be a primary diagnostic tool in the study of H\ensuremath{\downarrow} films.
Possible acoustic modes in a superfluid 2D gas of hydrogen atoms adsorbed on the surface of liquid helium at T ≲ 0.1 K are considered depending on the oscillation frequency and times of energy and momentum transfers both between 2D subsystems of hydrogen atoms and ripplons and into the bulk liquid or substrate. Analogues of the usual and second sounds are realized in 2D hydrogen at high frequencies. In the case of weak coupling with the bulk liquid and substrate, ripplons provide an addition to the normal hydrogen component, which leads to a change in the speed of the second sound. In the most interesting range of low frequencies, an analogue of the fourth sound is realized, when ripplons and the normal hydrogen component are immobile and only the superfluid hydrogen component moves. In this case, when the rate of heat transfer into the bulk liquid is much lower than the sound frequency, oscillations of the temperature of hydrogen can be observed in phase with density oscillations. Methods for exciting acoustic modes are discussed.
The probability of sticking of spin-polarised atomic hydrogen, H down arrow , on the superfluid helium surface as a function of temperature from 73 to 526 mK has been identified to arise from its interaction with third-sound ripplons. When the number density of the spin-polarised hydrogen atoms in the bulk helium becomes equal to that of the third-sound ripplons, the rate of sticking of H down arrow on the surface of the superfluid helium becomes a linear function of temperature. The predicted linearity is in accord with the experimental measurements.