The effect of 4He on the relaxation rates of the two-level systems in porous Vycor glass

Abstract

The sound speed in porous Vycor glass is strongly dependent on the amount of 4He adsorbed in the pores. This is due to the interaction between the Two-Level Systems (TLS) in the glass and excitations in the helium film. As helium is added, the TLS relaxation rates gradually increase, to reach a maximum at approximately 90% of ncrit, the critical coverage for the onset of superfluidity. Very close to ncrit a local minimum in the relaxation rates is found. A further increase in the coverage causes again an increase in the relaxation rates. For thicker films and filled pores, the relaxation rates become essentially independent of the amount of helium adsorbed. We then find a new behavior in which the relaxation contribution to the sound speed scales with temperature and frequency as T/ωl4. This indicates that an efficient relaxation channel becomes available for the TLS when there is superfluid in the pores. The rapid change in relaxation rates seen close to the completion of the “inert layer” indicates a change in excitation spectrum in the helium film at the onset of superfluidity.