Anharmonic Effects in Ultrasonic Propagation Near the λ-Transition of 4He
We have carried out the first measurements of second harmonics of a longitudinal ultrasonic wave generated in pressurized liquid 4He near the λ-transition. The measurements yield the behavior of the anharmonic coupling coefficient, C, which represents the non- linearity of the system, near the critical region. The importance of these anharmonic terms near the critical region for the understanding of critical phenomena has been stressed recently by Gitterman1.
KeywordsLithium Attenuation Acoustics Onic
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- 2.R. T. Beyer and S. V, Letcher, Physical Ultrasonics, (Academic Press, 1969 ), chapter 7; 0. V. Rudenko and S, I. Soluyan, Theoretical Foundation of Nonlinear Acoustics, ( Consultants Bureau, New York and London, 1977 ).Google Scholar
- 3.The hydrodynamic equations (1) and (2) are considered to be valid in the temperature range near the transition where the inequality qξ ≪ 1 is fulfilled. Here q is the wave number of the probe and ξ, is the correlation length of the fluctuations. For 10 MHz ultrasonic waves used as the fundamental wave in the present study, the above condition is satisfied up to |t| ≃ 10-5.Google Scholar