The sound attenuation and velocity change in pyrolytic graphite have been simultaneously measured as a function of the temperature, frequency and magnetic field intensity. It has been found that both the attenuation coefficient and the velocity change are greatly enhanced when some electron and hole Landau levels cross simultaneously the Fermi level at a certain field. The interrelation between the enhancement rate in the attenuation coefficient and that in the velocity change is reasonably explained in terms of the electron-hole correlation interaction proposed by Kuramoto. There remains, however, unexplained, as for the experimental results of rather strong temperature dependence of both quantities and weak frequency dependence of the velocity change.
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