Nonlinear acoustic properties of vibrating wires at very low temperatures

The authors have investigated the acoustic properties of superconducting NbTi and Ta wires in vacuum at 1 mK≤T≤1 K and at a few kHz. The temperature dependence of the acoustic properties of the wires is similar to that found in amorphous materials: a maximum in the sound velocity and a plateau in the dissipation at temperatures above it. In addition, they have observed a strong influence of the acoustic power on the measured properties in agreement with recent measurements on amorphous SiO2. This strain dependence can be interpreted by a modification of the tunneling model as a change of population S of the two-level system energy states in non-crystalline materials. They can explain the strain-dependent anomalies as the superposition of three nonlinear effects: the change of population of the tunneling systems energy states, self-heating of the wire, and a nonlinear restoring force. These effects have a strong influence on the lineshape of the resonance curve. In addition, they have investigated the behavior of vibrating wires in liquid 3He and liquid 3He-4He solutions at T<100 mK. They can show that in superfluid 3He-B at T<0.2 mK or in solutions of 3He in 4He with a small concentration of 3He it is impossible to use a vibrating wire as a viscometer without having exact information about its intrinsic properties. 32 refs., 12 figs., 1 tab