THERMODYNAMIC VELOCITY AND DISPERSION OF FIRST SOUND IN LIQUID-HELIUMNEAR THE LAMBDA-TRANSITION

We extend the functional form of thermodynamic velocity of first sound in liquid helium near lambda transition, at different pressures, by u sing the heat capacity (as proposed by Chase[1], Pippard[2] and G. Ahl ers[3]) to reach the hydrodynamic region ( / t / = / T-T-lambda / > 10 mK). For heat capacity we use both descriptions, logarithmic and powe r-law. These results are compared with experimental data obtained in r esonant cavities for 20 mu K < / t / < 1 mK and frequencies omega, les s than 7.5 KHz. In adition, these results are used to calculate the cr itical dispersion in the phenomenological theory of Ferrell and Bhatta charjee[4], based on dynamic scale theory. A strikingly good agreement with experimental results is found.