Comparison between torsional oscillator and ultrasonic measurements of 4He confined in nano-porous media

We have performed torsional oscillator and ultrasonic measurements for He-4 confined in 2D nano-porous substrate, hectorite. For He-4 films, the resonance frequency of the torsional oscillator increases rapidly just below a superfluid transition temperature (T (C) ), and then becomes constant at the lower temperature. In contrast, the sound velocity below T (C) increases monotonically with decreasing temperature. For liquid He-4 the sound velocity has a small bend at T (b) of about 0.9 K. Its increase below T (b) is enhanced as the pressure is increased.