The attenuation of first sound near the lambda point can be interpreted as being due to two phenomena: (1) a relaxation process described by Pokrovskii and Khalatnikov occurring only below the lambda point, and having a relaxation time of ξC₂ where C₂ is the velocity of second sound and ξ is a coherence length of magnitude 1.36×10^-8(T_λ-T)^-23 cm; (2) a critical attenuation which is nonsingular and symmetrical about T_λ.
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Williams et al. (1970) studied this question.
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