Amplitude-dependent ultrasonic attenuation in superconductors

An anomaly in the absorption of ultrasonic waves has been found in some very pure superconductors and and has been investigated in detail in superconducting lead. It consists in a strong dependence of the absorption on the amplitude of the ultrasonic waves. The amplitude dependence is only weakly present in the normal state. The effect is found to be strongly temperature-dependent, decreases as impurities are added to the crystal, and exhibits little or no change with frequency. Deformation and annealing have a pronounced effect on the anomaly. It appears that these characteristics can be understood in terms of a model, which is proposed here, based on the assumption of a strong interaction between the conduction electrons and the dislocations in the metal crystal. The presence of weakly pinned dislocations and their motion in the field of the sound wave is assumed. In the normal state, this motion is highly damped by the conduction electrons, and therefore, the dislocations cannot contribute much to the ultrasonic absorption. In the superconducting region, this damping decreases with the same temperature dependence as the ultrasonic absorption by the electrons. The dislocations become more free to move and therefore can cause an (amplitude-dependent) absorption by themselves. The mechanism for this absorption is thought to be dislocation unpinning similar to that discussed, for example, by Granato and Lücke.