Ultrasonic study of tunnelling defects in amorphous Se, Ge and Se-Ge compounds

We have measured, in the 100 MHz range and between 0-1K and 10 K, the velocity and the attenuation of ultrasonic waves propagating in amorphous Se, amorphous Ge and amorphous Se-Ge compounds. The results are explained in the framework of the theory of two-level systems. In particular, the existence of a two- level system in a tetracoordinated amorphous network is clearly revealed. Moreover, we report variations of the coupling parameter of the two-level system as a function of the rigidity of the network and explain these in terms of the density and the deformation potential of the two-level systems.