Surface acoustic wave investigation of hydrogen tunneling in niobium films at low temperatures

The tunneling of hydrogen in niobium at temperatures below 10 Kelvins is of great interest as it represents a model system in which a well known tunneling atom interacts with electrons and phonons. This tunneling system has been studied thoroughly in bulk samples of Nb:H. Prior results have been explained successfully with a "standard model" containing a narrow density of states, a low-energy cutoff, and a weak tunneling system-phonon coupling. The tunneling of hydrogen in thin polycrystalline films of Nb, however, has not been studied. We have used GHz surface acoustic waves to examine the properties of Nb:H films to see if the standard model applied to thin films. The changes in attenuation and velocity of the surface waves were measured from 0.1 K to 10 K. The standard model explains our results in a qualitative way. To improve agreement with the data, a small number of strongly-coupled tunneling systems must be added to the model. We also find indications that states may exist at energies excluded from the standard model.