Abstract
In a recent second-sound experiment in a dilute {sup 3}H-{sup 4}He mixture, Church et al. were able to understand most of their data with the theory by d'Humieres, Launay, and Libchaber. However, significant discrepancies exist at lower frequencies. The authors show that diffusive currents across the superleak are responsible for this deviation. What is more, they are able to determine the diffusion coefficient D from the low-frequency data. The equations governing the performance of oscillating superleak transducers and Peshkov transducers are derived in a systematic manner. This pertains to pure He-II as well as to mixtures. Mistakes in earlier works on dissipative effects of superleak transducers are corrected. Explicit formulas are given for the amplitude and linewidth of second sound, and the experimentally relevant parameters are discussed.
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