Nonlinear acoustic properties of vibrating wires at very low temperatures

The acoustic properties of superconducting NbTi and Ta wires have been investigated in vacuum at 1 mk < = T < = 1K 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, a strong influence of the acoustic power on the measured properties has been observed in agreement with recent measurements on amorphous SiO sub 2 . 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. The strain-dependent anomalies may be explained 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, the behavior of vibrating wires in liquid exp 3 He and liquid exp 3 He-- exp 4 He solutions at T < 100 mK was investigated. It is shown that in superfluid exp 3 He--B at T < 0.2 mK or in solutions of exp 3 He in exp 4 He with a small concentration of exp 3 He it is impossible to use a vibrating wire as a viscometer without having exact information about its intrinsic properties.