Excitonic Instability and Ultrasonic Attenuation in Strong Magnetic Fields

Anomalies in ultrasonic attenuation in Bi under strong magnetic fields discovered by Mase et al. are interpreted as arising from the fluctuations of the order parameter of the excitonic phase above the critical temperature. The critical temperature is estimated to be of the order of 1^∘{} and the critical exponent of the excess contributions to the attenuation coefficient is {}. These are in good agreement with the experiments.