Tunneling systems in polycrystalline metals: Absence of electron-assisted relaxation

With the vibrating-wire technique we have investigated the acoustic properties of polycrystalline Al and Ta in the normal and superconducting states and of Pt. The measurements were performed at frequencies 0.1 ≤ν≤18.5 kHz and at temperatures 0.1 mK ≤T≤10 K. The relative change of the sound velocity and the internal friction resemble those of amorphous dielectrics with similar strain dependence. No difference in the acoustic properties has been observed between the normal and superconducting states indicating the absence of electron-assisted relaxation of the tunneling systems. This result is in agreement with the results for Pt. In addition, we observed in Pt similar changes of the acoustic properties after thermal treatment as those reported for amorphous superconductors. These changes indicate a decrease of the coupling constant between phonons and tunneling systems upon heat treatment.