抄録
Anomalous behaviors of the ultrasonic attenuations in Bi under strong magnetic fields found by Mase et al. are analyzed by taking account of the excitonic instability. Characteristic features of anomalous growth of the peak value of the lower field side, in the case where the two peaks of the attenuation coefficient due to electrons and holes lie close as a function of magnetic field, can be attributed to the fluctuation of the order parameter of excitons above the critical point Tc, which is around 1 K. Critical exponent of the excess contributions from the fluctuation to the attenuation coefficient is one half, which is in accordance with the experiments.
