Effect of lead impurities on the amplitude-dependent absorption in normal and in superconductive indium

A study was made concerning the effect of small concentrations of lead impurities on the amplitude dependence of ultrasound absorption in normal and in superconductive indium. The frequency of longitudinal vibrations was 7.5 MHz. The single-crystal specimens for this study had all the same orientation. The ultrasonic waves were propagated in the [110] direction. Measurements were made at 1.4 and 77°K. The lead content was varied from 1 × 10−3 to 3 × 10−1 at.%. In each case the transition to the n- state was accompanied by a shift of the absorption curve toward larger amplitudes. With the lead content increasing up to 3 × 10−1 at.%, the critical amplitude had increased more than 103 times. Implantation of impurity atoms had produced an appreciable change not dependent on the amplitude but nonmonotonically dependent on the impurity content: It first decreased and then slightly increased with increasing lead content. Unlike the amplitude-dependent absorption, this amplitude-independent absorption did not change during the s-transition at lead contents higher than c = 5 × l0−2 at.%. These results indicate that the dynamic interaction between impurity atoms and dislocations contributes to the quasiviscous braking of the latter. As a result, the damping in doped single crystals does not change significantly during the s- transition. The change in conditions under which pinning centers are overcome by dislocations during the s- transition is attributed to a quasistatic mechanism.

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