EunJoo Thompson

Low temperature acoustic properties of amorphous silica and the tunneling model

Internal friction and speed of sound were measured on a-SiO2 above 6 mK using a torsional oscillator at 90 kHz, controlling for thermal decoupling, vibrational heating, background losses, and nonlinear effects. Strain amplitudes epsilon(A) = 10(-8) mark the transition between the linear and nonlinear regimes. In the former, agreement with the tunneling model was observed for both internal friction and speed of sound above 25 mK. The observed deviations in the speed of sound below 25 mK can be described with a cutoff energy of Delta(0, min)/k(B) = 6+/-0.5 mK. In the nonlinear regime, above 10 mK the behavior was typical for nonlinear harmonic oscillators, while below 10 mK different behavior was found.

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

The low-temperature internal friction Q−1 and thermal conductivity κ of plastically deformed, high-purity niobium monocrystals have been investigated and compared with measurements on an amorphous SiO2 (a-SiO2) 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-SiO2 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 1.5K, 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 T3 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.

Low‐Temperature Acoustic and Thermal Properties of Plastically Deformed, High‐Purity Polycrystalline Aluminum

The low-temperature internal friction Q -1 , thermal conductivity K, specific heat cp and heat release of plastically deformed, high-purity aluminum polycrystals have been investigated and have been compared with measurements on an amorphous SiO 2 specimen. Plastic deformation has a pronounced effect on both internal friction Q -1 and thermal conductivity K in the superconducting state. The magnitude of the internal friction Q -1 can be increased over two orders by plastic deformation over that observed on an annealed sample, and approaches a value approximately equal to that of the amorphous SiO 2 specimen. The lattice thermal conductivity K of the deformed specimens also has a magnitude which is of the same order as that of amorphous SiO 2 , it is, however, nearly independent of the amount of deformation. No glass-like anomalies could be observed in the specific heat cp and heat release measurements. The specific heat c p approaches a T 3 -relationship at the lowest temperatures investigated, and heat release experiments clearly show no long-time energy relaxation effects. Thus, it must be concluded that the defects introduced into deformed aluminum cannot be described with the tunneling model which had been proposed to describe the low temperature elastic and thermal properties of amorphous solids and which is based on the assumption of a constant spectral density of tunneling states. The phonon scattering mechanism observed in the deformed aluminum is tentatively related to the interaction of phonons with geometrical kinks in dislocations.

Dislocation Tunneling in Plastically Deformed Aluminum

We have measured the amplitude dependent internal friction (ADIF) and sound velocity down to 0.065 K of a 99.9999% (6N) aluminum that was plastically deformed by 1%. The strain amplitude was varied from e = 1 × 10 −8 to e = 6 × 10 −7 , a factor of 60. It is observed that with increasing strain amplitude, both the internal friction and the sound velocity become temperature independent over an increasingly wider temperature range. ADIF measurements show a flattening of the temperature dependence of the strain amplitude at low temperatures which is taken to be strong evidence for dislocation tunneling.

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 .