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Submitted on 1 Jan 1978
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THIRD SOUND MEASUREMENTS IN 3He-4He
MIXTURES USING TIME OF FLIGHT
TECHNIQUES
J. Brooks, R. Hallock
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 8 , Tome 39, aozit 1978, page C6-314
THIRD SOUNE MEASUREMENTS
I N
3He-4He MIXTURES USINGTIME
OF FLIGHT TECHNIQUES?J.S. Brooks and R.B. Hallock
tt
Department of Physics and Astronomy University of Massachusetts Amherst, Massachusetts 01003, USA
Rdsum6.- Nous prdsentons nos premiers rdsultatsexpdrimentaux de vitesse du troisisme son dans un mdlange 3 ~ e - 4 ~ e . A proportions dgales de 3 ~ e et 'He dans notre systsme, la vitesse prdsente une ddpendance inattendue en tempdrature. Les expdriences effectudes sur 'He pur dans le mzme systSme ne pr6sentent aucune ddpendance anormale.
Abstract.- We report preliminary measurements of the velocity of third sound in 3 ~ e - 4 ~ e mixtures. With equal amounts of 3 ~ e and 'He sealed into the apparatus the velocity displays an unexpected temperature dependence and for part of the range is independent of the film thickness. Experiments on pure 4 ~ e in the same apparatus show no anomalous behavior.
There exists in the literature a large number of experiments involving third sound in pure 'He. Much less information is available on third sound in
3 ~ e - 4 ~ e mixtures /1,3/. The present measurements were motivated in part by a desire to study the phenomena of third sound and phase separation in detail in mix- ture helium films. We report here preliminary measu- rements on 3~e-'~e mixtures where equal amounts of 3 ~ e and 'He were sealed into the apparatus. We also present results for pure 4 ~ e for comparison.
The apparatus used for this work consists of
an experimental chamber which contains the third sound time of flight apparatus /4/ and A1203 powder to provide a large reservoir of superfluid film. The chamber can be sealed at low temperature by means of a superfluid value. The procedure followed is to seal a known amount, Ai, of helium into the chamber at 0.4 K. In the case of mixtures we have used equal amounts, Ai/2, of 3 ~ e and 'He. Once the selected amount of helium Ai is sealed into the apparatus, third sound is generated and its ;elocity measured by time of flight techniques. Measurements are ta- ken at selected temperatures as the apparatus is warmed from 0.4 K. Data taken during warming
or
cooling is reproducible. Duri'ng the warning atoms evaporate from the film into the dead volume o f the apparatus and the film necessarily thins, Since forpure 4 ~ e /5/
4 Supported by the National Science 'Foundation D?IR 76-08260
'f'f On leave until September 1978, LASSP, Cornell University, Ithaca, New York.
where f = 3a/d4
,
a thinner film results in a hi- gher velocity of third sound. Here, Cg is the third sound velocity, <p >/p the reduced superfluid frac-tion in a film of thickness d, f is the van der Waals force with van der Waals constant
a,
T the temperature, S the entropy and L the latent heat. Examples of our observations of C 3 vs temperature are shown in figure 1 where the effect of the thin- ning film above 1.0 K is readily evident for theTEMPERATURE 1K)
Fig, 1 : Third sound velocity vs temperature for pure 'He. The expected effect of the eva- poration of the film into the sealed cell dead volume is shown by the solid lines
thinnest film studled, Also shown on the figure (solid lines) is the expected effect of thls thin- ning on C3 fitted to the surface area and known dead
volume of our apparatus. At 0.4 K the amounts of helium, A.
,
introduced into the apparatus in the three cases shown resulted in film thickness values of 8.6, 11.5 and 13.4 atomic layers. For the caseof pure 4 ~ e the temperature dependence of the third sound velocity is seen to be in agreement with expectations.
The behavior is radically different for the case of mixtures 161. Some of our results are shown in figure 2 for some of the same amounts of helium, Ai as in the case of the results in figure 1 except in this case a fraction of helium Ai/2 consisted of 3 ~ e . At low temperatures the amount of 'He contained
0
0.4 0.6 0.8 1.0 1.2 TEMPERATURE (K)
Fig. 2 : Third sound velocity vs temperature for the 3 ~ e - 4 ~ e mixtures as described in the text. Common symbols in figs. 1 and 2 refer to common
total
amounts of helium sealed into the apparatusin the mixture film represented by the open circles in figure 2 was the same as the amount of '~e contai- ned in the pure '~e film represented by the triangles in figure 1. In each mixture case there is a local maximum in the third sound velocity at a temperature
near that of phase separation in a bulk 3 ~ e - 4 ~ e mix- ture with a concentration of 50%. The location of this peak generally independent of Ai for the range of values studied to date (i = 1-7). At higher tem- peratures the third sound velocity decreases and
follows a universal curve independent of the value of Ai. The third sound velocity is observed to reach a minimum whose location depends on Ai. At tempera- tures above those at which the minima are located the third sound velocity is a strongly increasing func- tion of temperature.
Under the assumption that different amounts of helium, Ai, introduced into the apparatus produce different thicknesses of helium film, we are led to the conclusion that the velocity of third sound in the mixture is independent of thickness in the region between the maximum and the minima and therefore is determined by forces which overwhelm the usual van der Waals force.
References
/I/ Ratn~m, B., and Mochel, J., Proc. 13th Intl. Conf. on Low Temp.Phys. (ed. K.D. Timerhaus et al, Plenum, New York, 1974)Vo1.1, p. 233
/2/ Downs, J., Ph. D Dissertation, (unpublished) USC, 1975 ; J. Downs and Kagiwada, R. S., Bull., Am.Phys.Soc.
17
(1972) 19/ 3 / Bergman, D.J., Pysical Acoustics (Academic Press, New York, 19751, Ch. 1
,
p. 49 / 4 / Brooks, J.S., Ellis, B.M. and Hallock, R.B.,Phys.Rev.Lett.
40
(1978) 240/ 5 / Bergman, D.J., Phys.Rev.
