Low-Temperature Internal Friction and Thermal Conductivity of Plastically Deformed, High-Purity Monocrystalline Niobium

The low-temperature internal friction Q^-1 and thermal conductivity of plastically deformed, high-purity niobium monocrystals have been investigated and compared with measurements on an amorphous SiO_2 (a-SiO_2) specimen. After plastic deformation at intermediate temperatures, an approximately temperature independent internal friction Q^-1 was observed with a magnitude comparable to that of the a-SiO_2 specimen. Plastic deformation at low temperatures leads to an internal friction Q^-1 with a considerably smaller magnitude. In the temperature range between about 0.3 and 1.5 K, the lattice thermal conductivity k of the deformed specimens decreases with increasing deformation. It is, however, nearly independent of the amount of deformation at the lowest temperatures investigated. In this temperature regime, the lattice thermal conductivity of the specimens varies proportional to T^3 and has a magnitude as would be expected for an undeformed sample. Additional heat release experiments on an undeformed sample clearly show no long-time energy relaxation effects. We conclude that the defects introduced by plastic deformation cannot be described with the tunneling model which had been proposed to describe the low temperature elastic and thermal properties of amorphous solids. The phonon scattering mechanisms observed in deformed niobium are tentatively related to the dynamic interaction of phonons with geometrical kinks in dislocations.