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

Quantitative experimental results of the second-sound damping $D_2$ near the superfluid transition temperature $T_{\lambda }\mathrm{}\left(P\right)\mathrm{}$ are presented at several pressures $P$ as a function of the reduced temperature $t=1-\frac{T}{T_{\lambda }\mathrm{}\left(P\right)\mathrm{}}$. The data cover the entire pressure range of the transition and are for $2\times {10}^{-5\mathrm{}}\lesssim t\lesssim 0.1$. 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 $T_{\lambda }$. The intricate pressure dependence of the data, which changes sign near $t\approx {10}^{-3\mathrm{}}$, 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.