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$^{4}$He Films on Grafoil Foam
New Phys.: Sae Mulli 2019; 69: 1123~1128
Published online November 29, 2019;
© 2019 New Physics: Sae Mulli.

Sang-Tack NAM*

Department of Physics and Research Institute of Natural Science, Gyeongsang National University, Jinju, Gyeongnam 52828, Korea
Correspondence to:
Received July 22, 2019; Revised September 9, 2019; Accepted September 17, 2019.
cc This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial License ( which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
We present the third sound-speed of liquid $ ^{4}$He films on Grafoil foam substrates as functions of the temperature and the film depth, which we obtained by using the hydrodynamic method of Rutledge \textit{et al}.. We also calculated the superfluid parameters $\overline{\rho}_{s}/\rho$ and $\overline{\rho}_{s}d/T$ of the liquid $ ^{4}$He films on Grafoil foam substrates by using the measured values of the third sound-speed, the van der Waals constant, and the thickness of the film. We found that the parameters of $\overline{\rho}_{s}/\rho$ and $\overline{\rho}_{s}d/T$ in liquid $ ^{4}$He film on each Grafoil foam substrate at its critical temperature were between 0.215 and 0.267 and between $3.46\times10^{-9}$ g/cm$ ^{2}$K and $4.32\times 10^{-9}$ g/cm$ ^{2}$K which had nothing to do with the film's thickness. The third sound-speed for the films on each substrate at its critical temperature was found to be decrease with increasing thickness of the film. Also, the value of the complementary factor in the equation representing the film coverage and the third sound-speed as functions of the thickness and critical temperature of liquid $ ^{4}$He films on Grafoil foam substrates seemed to be negative or small for a thin film at its low critical temperature, but it was likely to be much large, for a thick film at its high critical temperature.
PACS numbers: 67.25 bh, 67.25 dt
Keywords: Grafoil foam substrate, Liquid $^{4}$He film, Third sound-speed, Superfluid parameter

November 2019, 69 (11)
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