Helium at nonzero temperatures: A variational density-matrix approach for Bose fluids

A variational statistical mechanics of boson quantum fluids such as He4 is developed as a natural generalization of the Jastrow Euler-Lagrange method which has been successfully applied to the ground-state problem. The results include a quantitative description of the liquid-gas portion of the phase diagram, including the corresponding critical point and spinodal line. Also calculated is the temperature dependence of the liquid structure function and phonon-roton spectrum within the class of trial density matrices explored in this work.