Path integral Monte Carlo determination of the fourth-order virial coefficient for unitary two-component Fermi gas with zero-range interactions

TL;DR

This paper uses a specialized path integral Monte Carlo method to accurately determine the fourth-order virial coefficient of a strongly-interacting unitary Fermi gas, resolving previous theoretical disagreements and aligning with experimental data.

Contribution

It introduces a customized ab initio PIMC algorithm with on-the-fly anti-symmetrization for zero-range interactions, providing a reliable way to compute virial coefficients in Fermi systems.

Findings

The calculated $b_4$ agrees with experimental measurements.

The method avoids Thomas collapse through on-the-fly anti-symmetrization.

Efficient treatment of small Fermi systems with zero-range interactions.

Abstract

The unitary equal-mass Fermi gas with zero-range interactions constitutes a paradigmatic model system that is relevant to atomic, condensed matter, nuclear, particle, and astro physics. This work determines the fourth-order virial coefficient $b_4$ of such a strongly-interacting Fermi gas using a customized \textit{ab initio} path integral Monte Carlo (PIMC) algorithm. In contrast to earlier theoretical results, which disagreed on the sign and magnitude of $b_4$, our $b_4$ agrees within error bars with the experimentally determined value, thereby resolving an ongoing literature debate. Utilizing a trap regulator, our PIMC approach determines the fourth-order virial coefficient by directly sampling the partition function. An on-the-fly anti-symmetrization avoids the Thomas collapse and, combined with the use of the exact two-body zero-range propagator, establishes an efficient general means to treat small Fermi systems with zero-range interactions.