Ultrasonic attenuation via energy diffusion channel in disordered conductors

We predict an existence of an new dissipation channel leading to attenuation of ultrasound in disordered conductors and superconductors with perfect electroneutrality. It is due to slow diffusion of thermal energy. We show that in doped silicon ultrasound attenuation may be enhanced by a factor about 100. Similar effect is also studied for s-wave and d-wave superconductors. The latter case is applied to BSCCO family where strong enhancement of ultrasound attenuation is predicted. For usual s-wave superconductors new dissipation channel might be important for very low-electron-density materials near the BCS-BEC crossover.