The free surface of superfluid 4He at zero temperature

TL;DR

This study uses diffusion Monte Carlo simulations to analyze the structure, energetics, and quantum properties of the free surface of superfluid helium-4 at zero temperature, providing improved estimates and detailed profiles.

Contribution

It extends previous Monte Carlo studies by using larger particle numbers and updated potentials to accurately characterize the superfluid helium surface.

Findings

Surface tension close to experimental values

Density profiles agree with recent data

Enhanced condensate fraction near the surface

Abstract

The structure and energetics of the free surface of superfluid $^4$He are studied using the diffusion Monte Carlo method. Extending a previous calculation by Vall\'es and Schmidt, which used the Green's function Monte Carlo method, we study the surface of liquid $^4$He within a slab geometry using a larger number of particles in the slab and an updated interatomic potential. The surface tension is accurately estimated from the energy of slabs of increasing surface density and its value is close to one of the two existing experimental values. Results for the density profiles allow for the calculation of the surface width which shows an overall agreement with recent experimental data. The dependence on the transverse direction to the surface of other properties such as the two-body radial distribution function, structure factor, and one-body density matrix is also studied. The condensate fraction, extracted from the asymptotic behavior of the one-body density matrix, shows an unambiguous enhancement when approaching the surface.