Superfluid Transition of a 4He Thin Film Pressurized by Bulk Liquid 3He
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We measured transverse acoustic impedance Z of normal fluid 3He at 46.6 MHz on a surface coated with a thin 4He film. The real component of the impedance, Z′, in the coated samples deviates from Z′ in the pure 3He in the low temperature region. Z′ on the coated samples is almost identical with Z′ in the pure sample at high temperature and gradually deviates below a particular temperature T onset . T onset is possibly the superfluid onset temperature of the 4He film pressurized by the bulk liquid 3He. The gradual decrease in Z′ means that the superfluid component in 4He film increases gradually, which is expected from the dynamic KT transition at high frequency. The thicker is the film, the higher is the T onset . The range of T onset we observed was between 40 and 160 mK. This is much lower than that at the saturated vapor pressure. Suppression of T onset achieved by the applied pressure from bulk liquid 3He was presumably caused by the dissolved 3He in the film, thickening of the inert layers and/or by the strong correlation effect. The result shows that the specularity of 3He quasiparticle scattering is strongly affected by superfluidity of the 4He film.