Damping of second sound in superfluid helium nearTλ

The damping coefficient of second sound has been measured in liquid helium at saturated vapor pressure near $T_{\lambda }$. The results cover the temperature range $40 \mathrm{mK}>T_{\lambda }-T>\mathrm{}40\mathrm{} \mu \mathrm{K}$, and were obtained by the measurement of decay rates in a high-$Q$ second-sound resonant cavity. These values are in good agreement with the predictions of nonlinear renormalization-group analysis applied to the asymmetric planar spin model for liquid helium in the "crossover" and precritical temperature ranges.