High-amplitude ultrasonic oscillations and dislocation unpinning in lead at 4.2°K

We investigated the amplitude dependence of the internal friction in lead single crystals without impurities and with 5·10−3 at.% Sn or Sb. The measurements were made at a longitudinal oscillation frequency of 7.5 MHz at 4.2° K. In measurements of the friction vs amplitude the direct run did not coincide with the back run, the result being a hysteresis loop with the back run curve below the direct run curve. The hysteresis loop grew larger with the amplitude reached at the end of the direct run and was much larger in the superconducting state than in the normal state. We studied the kinetics of the changes in the internal friction under the effect of high-amplitude ultrasonic oscillations. The results are analyzed in the light of the Granato–Lücke and Rodgers athermal theories and indicate that the structure of the pinning points changes under multiple dislocation unpinning as the result of alternating high-amplitude stress.