Search of superfluidity of solid 4He in a porous vycor glass by means of the ultrasound technique

Kim and Chan have reported that solid He-4 shows a reduction of the rotational moment of inertia below 0.2 K, which suggests an onset of superfluidity. Ultrasound should be sensitive to mass decoupling caused by superfluidity. If a superfluid component exists, the sound velocity of a porous material filled with solid 4He could increase. We have carried out ultrasonic measurements for a porous Vycor glass filled with solid 4He. Since the reported "critical velocity" is very low, we have adopted a continuous wave resonance technique which realizes the oscillating velocity less than 1 X 10(-7) m/s. The resolution of the sound velocity is 10(-5) for small oscillating velocities, and is enough to detect the expected mass decoupling. Although the present experimental conditions are rather limited, no signature of supersolid has been observed.