EXCITED-STATES IN SUPERFLUID HE-4 - A MONTE-CARLO APPROACH

We present diffusion Monte Carlo calculations of the phonon-roton spec trum and the vortex excitation in liquid He-4. The sign problem associ ated with the excited state is solved in the framework of the fixed no de approximation. Within this approximation it is shown that DMC impro ves in a significant way the well known variational predictions for th e phonon-roton spectrum. Results for the core energy and density profi le of a 2D vortex are also presented with an efficient method to work out the infinite extension of the phase present in the Feynman wave fu nction. Finally, we show how so the fixed node bounds are improved tow ards the exact values when the nodal surface is released.