Precise determination of the structure factor and contact in a unitary Fermi gas

TL;DR

This paper precisely measures the universal contact parameter in a strongly interacting Fermi gas at unitarity using Bragg spectroscopy, providing a benchmark for zero-temperature homogeneous systems and validating quantum Monte Carlo calculations.

Contribution

It offers the first high-precision experimental determination of the contact in a trapped Fermi gas at unitarity and compares it with advanced quantum Monte Carlo simulations.

Findings

Contact value of 3.06 ± 0.08 at 0.08 T_F

Agreement between experiment and Monte Carlo within a few percent

Provides a new benchmark for zero-temperature homogeneous contact

Abstract

We present a high-precision determination of the universal contact parameter in a strongly interacting Fermi gas. In a trapped gas at unitarity we find the contact to be $3.06 \pm 0.08$ at a temperature of 0.08 of the Fermi temperature in a harmonic trap. The contact governs the high-momentum (short-range) properties of these systems and this low temperature measurement provides a new benchmark for the zero temperature homogeneous contact. The experimental measurement utilises Bragg spectroscopy to obtain the dynamic and static structure factors of ultracold Fermi gases at high momentum in the unitarity and molecular Bose-Einstein condensate (BEC) regimes. We have also performed quantum Monte Carlo calculations of the static properties, extending from the weakly coupled Bardeen-Cooper-Schrieffer (BCS) regime to the strongly coupled BEC case, which show agreement with experiment at the level of a few percent.