THE SURFACE OF LIQUID-HE-4 AT NONZERO TEMPERATURES

We outline a general approach to microscopic evaluation of the propert ies of strongly interacting, spatially inhomogeneous Bose systems at f inite temperatures. A minimum principle for the Helmholtz free energy is used together with an;appropriate trial density matrix to generaliz e the correlated variational wave function theory that has proven so s uccessful in the treatment of the ground states and elementary excitat ions of quantum fluids at zero temperature. Euler-Lagrange equations a re obtained that determine the optimal structure through the one-and t wo-body densities and the optimal density fluctuation operators and en ergies characterizing the elementary excitations, Some results of an a pplication of this correlated density matrix theory to the He-4 liquid -vapor interface are presented, with particular focus on the character ization of resonant vapor modes.