The resonant frequency of a quartz crystal oscillating in the thickness-shear mode changes by virtue of the mass loading due to any type of film laid down on its surfaces. For a helium film only the normal fraction remains rigidly coupled to the substrate motion. Adsorption isotherms of He⁴ measured with this technique show clear departures from conventional nonsuperfluid behavior. We attribute this effect to the onset and presence of superfluidity in the film.
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Chester et al. (1972) studied this question.
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