TEMPERATURE-DEPENDENCE OF THE SURFACE-TENSION OF LIQUID-HE-4

Recent calculations of the surface excitations of liquid He-4 show tha t besides the ripplon branch, a second branch of excitations, with one node in the transition density, exists at higher energy. Back-flow ef fects are essential for a quantitative description. Inclusion of both branches in the thermodynamics of the surface provides good agreement with experiment for the temperature dependence of the surface tension sigma(T) up to the lambda-transition. Above T-lambda, the variation of sigma(T) is mainly governed by the difference between the liquid and vapor densities.