Ultrasonic attenuation in clean d-wave superconductors

We consider the attenuation of longitudinal ultrasonic waves in a clean two-dimensional d-wave superconductor. We show that the attenuation coefficient is linear in temperature at low temperatures for all in-plane directions of the propagation of the ultrasound, and that the coefficient of the linear term can be used to determine the parameters crucial for the low temperature transport in these compounds.