Dislocation drag in sodium chloride at low temperature—A radiation-damping model

The contribution of dislocations to ultrasonic attenuation in the liquid-helium temperature range has been measured in sodium chloride for the purpose of determining the resistive force acting on dislocations. Use was made of a technique for measuring the ultrasonic attenuation change $\Delta \alpha$ at different frequencies, caused by a bias stress. The predictions of the extensible-string model of dislocations, which account well for the behavior of $\Delta \alpha$ above about 70°K, are not consistent with the observed frequency and amplitude dependence of $\Delta \alpha$ at lower temperatures. A dislocation-drag model based on a radiation-damping mechanism is shown to account for the present results. Furthermore, this mechanism, taken in conjunction with the viscous damping normally assumed for the extensible-string model, also accounts qualitatively for the behavior of dislocation damping at low frequencies (kHz), which shows discrepancies with the string model.