The Acoustic Impedance of Liquid Helium-3

Measurements have been made of the acoustic impedance (Z) of liquid 3He under its saturated vapour pressure in the temperature range 0·035 to 0·6°K. A 1000 Mc/s sound wave is propagated along the axis of an X-cut quartz crystal, and undergoes many reflexions from each end of the crystal. Values are obtained for the reflexion coefficient at an interface between quartz and liquid 3He by comparing the rate at which the signal decays in the crystal, with and without liquid 3He present on the ends. These values then lead directly to the acoustic impedance (Z) of the liquid. Usually, Z/ρ (where ρ is the density) should be equal to the velocity of sound, and above 0·1°K this is found to be so. However, at about 0·1°K the value of Z/ρ increases abruptly, and at lower temperatures has a value about 10% greater. This result is in good agreement with Landau’s theory of a Fermi liquid, which relates the change in impedance with the propagation of a new mode of sound, the so-called 'zero-sound’.