Damping of second sound near the superfluid transition ofHe4as a function of pressure

Abstract

Quantitative experimental results of the second-sound damping ${D}_{2}$ near the superfluid transition temperature ${T}_{\ensuremath{\lambda}}(P)$ are presented at several pressures $P$ as a function of the reduced temperature $t=1\ensuremath{-}\frac{T}{{T}_{\ensuremath{\lambda}}(P)}$. The data cover the entire pressure range of the transition and are for $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}\ensuremath{\lesssim}t\ensuremath{\lesssim}0.1$. Their experimental uncertainty is in the range of 2-4%. The experimental technique used in this work is described in detail. It is based on a tone-burst method and includes a number of novel features such as discrimination against higher harmonics by spectral analysis of the pulses, rectification before signal averaging to avoid the detrimental effect of temperature noise, a careful study of finite-amplitude effects, and quantitative corrections for nonparallelism of the cavity ends. The results are compared with the predictions based on renormalization-group theory and thermal-conductivity measurements above ${T}_{\ensuremath{\lambda}}$. The intricate pressure dependence of the data, which changes sign near $t\ensuremath{\approx}{10}^{\ensuremath{-}3}$, is given rather well by the prediction. However, the details of the temperature dependence of the data at a given pressure disagree with the theory in its present form by deviations somewhat greater than the experimental uncertainty.