Condensate fraction in liquid 4He at zero temperature

TL;DR

This paper uses Path Integral Ground State Monte Carlo methods to accurately calculate the condensate fraction and kinetic energy of liquid helium-4 at zero temperature, showing good agreement with experiments and exploring overpressurized states.

Contribution

It provides a model-independent, highly accurate calculation of the condensate fraction and kinetic energy of liquid helium-4 at zero temperature using unbiased Monte Carlo methods.

Findings

Condensate fraction decreases with pressure, reaching 0.8% at 87 bar.

Results agree well with recent experimental data.

Overpressurized liquid helium-4 maintains a small but finite condensate fraction.

Abstract

We present results of the one-body density matrix (OBDM) and the condensate fraction n_0 of liquid 4He calculated at zero temperature by means of the Path Integral Ground State Monte Carlo method. This technique allows to generate a highly accurate approximation for the ground state wave function Psi_0 in a totally model-independent way, that depends only on the Hamiltonian of the system and on the symmetry properties of Psi_0. With this unbiased estimation of the OBDM, we obtain precise results for the condensate fraction n_0 and the kinetic energy K of the system. The dependence of n_0 with the pressure shows an excellent agreement of our results with recent experimental measurements. Above the melting pressure, overpressurized liquid 4He shows a small condensate fraction that has dropped to 0.8% at the highest pressure of p = 87 bar.