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 s ystem (TLS) density of states and a one-phonon-TLS coupling strength v ery similar to a-SiO2. In addition, because of the sharply reduced sou nd speed in Vycor, the effect of the first-order Raman process is clea rly 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 l ower temperature of the relaxation contribution to the changes in the sound speed and the ultrasonic attenuation. The effect is strongly cov erage dependent. This can be most clearly observed in the ultrasonic a ttenuation 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 sim ilar behavior for low n, but the attenuation reaches a maximum for n c lose to n(c), the critical coverage for the onset of superfluidity. Fo r higher coverage it drops back to approximately half its maximum valu e. With completely filled pores the contribution to the sound speed sh ows a clear scaling with T4/omega, in contrast to the T3/omega or T7/o mega behavior expected for the direct and Raman relaxation processes.