The variation of the velocity of Rayleigh waves up to 20 MHz has been measured down to 0.1K in a thick film (20 mu m) of amorphous germanium. The logarithmic dependence of the velocity then observed is evidence for the existence of tunneling defects (two-level systems) in a four-coordinated amorphous network.
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Duquesne et al. (1983) studied this question.