Surface acoustic wave propagation in Nb:H at low temperatures

The acoustic properties of evaporated polycrystalline niobium films have been studied to temperatures below 100 mK using surface acoustic waves. The experiments probe the process which occurs when hydrogen atoms tunnel between two nearly degenerate interstitial sites. The interaction of sound with tunneling defects leads to temperature dependent changes in acoustic absorption and velocity below 10 K. By analyzing the acoustic properties, we extract the tunneling system relaxation rates and their coupling strengths to electrons and phonons. The presence of the superconducting gap below T_c inhibits relaxation through the electronic channel allowing us to separate electronic and phononic decay paths