Low-temperature internal friction in high-purity monocrystalline and impure polycrystalline niobium after plastic deformation

The internal friction Q(-1) of plastically deformed, high-purity monocrystalline and impure polycrystalline niobium specimens was measured in the temperature range between 65 mK and about 2 K. Plastic deformation has a pronounced effect on the internal friction Q(-1) of the high-purity monocrystalline specimens, and the effect has been found to be almost temperature independent. By contrast, surprisingly the internal friction Q(-1) of the impure polycrystalline specimens was found to be almost independent of the extent of plastic deformation. Comparison of the experimental results with different models of a dynamic scattering of acoustic phonons by dislocations leads to the conclusion that the results cannot be explained with the two-level tunneling model. Instead it is suggested that a strong interaction between acoustic phonons and geometrical kinks in non-screw dislocations is responsible for the observed internal friction Q(-1).