Acoustic behavior of the amorphous superconductor Cu 60Zr 40 at very low temperatures

We report measurements of the acoustic behavior of amorphous superconducting Cu ₆₀Zr ₄₀ at temperatures between 90 mK and 2 K. We observe a discontinuity in the slopes of the absorption as well as of the sound velocity at the transition temperature ( T_c) which can be very well explained by the tunneling -model. We also observe saturation of the -acoustic absorption when the sample is in the superconducting state.