NONLINEAR ACOUSTIC PROPERTIES OF VIBRATING WIRES AT VERY LOW-TEMPERATURES

We have investigated the acoustic properties of superconducting NbTi and Ta wires in vacuum at 1 mK less-than-or-equal-to T less-than-or-equal-to 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, we 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. We 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, we have investigated the behavior of vibrating wires in liquid He-3 and liquid He-3-He-4 solutions at T < 100 mK. We can show that in superfluid He-3-B at T < 0.2 mK or in solutions of He-3 in He-4 with a small concentration of He-3 it is impossible to use a vibrating wire as a viscometer without having exact information about its intrinsic properties.