DIFFUSION MONTE-CARLO FOR EXCITED-STATES - PHONONS AND ROTONS IN SUPERFLUID HE-4

We have applied the fixed-node and the released-node Monte Carlo techn iques to calculate the phonon-roton excitation spectrum (epsilon(q)) o f superfluid He-4. An excellent agreement with the experimental spectr um is achieved both at the equilibrium and near the freezing densities . The strength of the single excitation, peak Z(q) in. the dynamic str ucture function S(q,omega) shows an overall agreement with experimenta l data. New results for epsilon(q) and Z(q) at a negative pressure clo se to the spinodal point are also reported.