EFFECT OF ADSORPTION OF HE-3 AND HE-4 ON THE LOW-TEMPERATURE ULTRASONIC PROPERTIES OF POROUS VYCOR

Ultrasonic measurements show that porous Vycor glass has a two-level system (TLS) density of states and a one-phonon-TLS coupling strength very similar to a-SiO2. In addition, because of the sharply reduced sound speed in Vycor, the effect of the first-order Raman process is clearly observable. Introduction of He-3 and He-4 in the pore space causes an increase of the TLS relaxation rates. This results in a shift to lower temperature of the relaxation contribution to the changes in the sound speed and the ultrasonic attenuation. The effect is strongly coverage dependent. This can be most clearly observed in the ultrasonic attenuation around 100 mK. With He-3 we observe a gradual increase with coverage n, which saturates for n > 22 mumol/m2. With He-4 we see similar behavior for low n, but the attenuation reaches a maximum for n close to n(c), the critical coverage for the onset of superfluidity. For higher coverage it drops back to approximately half its maximum value. With completely filled pores the contribution to the sound speed shows a clear scaling with T4/omega, in contrast to the T3/omega or T7/omega behavior expected for the direct and Raman relaxation processes.