Microscopic calculation of the phonon-roton branch in superfluid 4-He

Departament de Física i Enginyeria Nuclear, Campus Nord B4-B5, Universitat Politècnica de Catalunya - E-08028 Barcelona, Spain

Received: 5 February 1997
Accepted: 17 March 1997

Abstract

Diffusion Monte Carlo results for the phonon-roton excitation branch in bulk liquid at zero temperature are presented. The sign problem associated to the excited wave function has been dealt both with the fixed-node approximation and the released-node technique. The upper bounds provided by the fixed-node approximation are shown to become exact when using the released-node method. An excellent agreement with experimental data is achieved both at the equilibrium and near the freezing densities. The leading order correction beyond the Feynman-Cohen excited wave function shows the presence of three-body correlations in the excitation operator.