High-frequency dislocation internal friction in bismuth single crystals at 4.2-300° K

Dislocation internal friction has been studied as a function of temperature (4.2–300°K), frequency (7.5–142.5 MHz), and amplitude. The temperature dependences indicate that dislocations contribute significantly to ultrasound absorption. An analytical method for separating loss mechanisms allowed us to find the frequency profile of overdamped dislocation resonance and to obtain the damping coefficient (B =3.1·10−4 dyn·sec·cm−2 at 300°K) for the {111} <101> dislocation system. At 4.2°K the amplitude dependence of absorption at the frequency of 7.5 MHz in the small-amplitude region, followed by a rising portion which goes over smoothly into a long plateau. When large-amplitude oscillations were excited absorption decreased in the amplitude-dependent region but remained unchanged in the amplitudeindependent region. In the amplitude-dependent region the absorption was found to depend on the number of large-amplitude pulses. The amplitude dependence at 4.2 °K is due to a thermal unpinning of dislocations; this makes it reasonable to compare the measured results with the predictions of the Granato-Liicke and Rodgers theories. The comparison lent support to these theories. As in the case of amplitude-independent losses, the absorption in the range of amplitudes near the maximum in the experiments is attributed to dynamic-type dislocation losses. In both cases these losses are determined by the same value of the damping coefficient but they increase sharply in the large-amplitude range owing to a power-law dependence on the dislocation-segment length which increases due to break-away from the pinning points. The bond strengths before and after excitation of large-amplitude oscillations were found to be in a 1:1.19 ratio.