Dispersion of First Sound in Liquid Helium near theλTransition

Abstract

The velocity of first sound in liquid helium has been measured for frequencies between 5.4 and 208 kHz and at temperatures $1 \mu \mathrm{K}<|T_{\lambda }-T|<\mathrm{}5\mathrm{} \mathrm{mK}$. The dispersion $u\left(\omega \right)-u\left(0\right)\mathrm{}$ for each frequency shows a single asymmetric peak at $T_{\lambda }$. The Landau-Khalatnikov order-parameter relaxation process alone is not adequate to describe the magnitude or the shape of the dispersion. The maximum of the dispersion remains frequency dependent even at the highest frequencies.