Ultrasonic attenuation due to resonant interaction with a distribution of level splittings of the ground state of shallow acceptors in Ge

The ultrasonic attenuation in Ge (Ga, In) has been measured in the frequency range from 500 MHz to 2.5 GHz, and from room temperature down to 1 K. Below 10 K the attenuation rises as ω²/T. For the first time saturation of the attenuation has been observed for ground state of a shallow acceptor. These results can be interpreted as due to resonance interaction with level splittings of a broad distribution with width of about 0.1 meV.