Density-wave ordering in a unitary Fermi gas with photon-mediated interactions

TL;DR

This study demonstrates the creation and control of density-wave order in a Fermi gas with tunable contact and photon-mediated long-range interactions, revealing their interplay across the BCS-BEC crossover.

Contribution

It introduces a fully tunable platform combining contact and long-range interactions in a Fermi gas, enabling detailed exploration of density-wave order and superfluidity interplay.

Findings

Density-wave order is stabilized above a critical long-range interaction strength.

The onset of density-wave order varies with contact interactions across the BCS-BEC crossover.

Atomic density-wave susceptibility can be tuned over an order of magnitude.

Abstract

A density wave (DW) is a fundamental type of long-range order in quantum matter tied to self-organization into a crystalline structure. The interplay of DW order with superfluidity can lead to complex scenarios that pose a great challenge to theoretical analysis. In the last decades, tunable quantum Fermi gases have served as model systems for exploring the physics of strongly interacting fermions, including most notably magnetic ordering, pairing and superfluidity, and the crossover from a Bardeen-Cooper-Schrieffer (BCS) superfluid to a Bose-Einstein condensate (BEC). Here, we realize a Fermi gas featuring both strong, tunable contact interactions and photon-mediated, spatially structured long-range interactions in a transversely driven high-finesse optical cavity. Above a critical long-range interaction strength DW order is stabilized in the system, which we identify via its superradiant light scattering properties. We quantitatively measure the variation of the onset of DW order as the contact interaction is varied across the BCS-BEC crossover, in qualitative agreement with a mean-field theory. The atomic DW susceptibility varies over an order of magnitude upon tuning the strength and the sign of the long-range interactions below the self-ordering threshold, demonstrating independent and simultaneous control over the contact and long-range interactions. Therefore, our experimental setup provides a fully tunable and microscopically controllable platform for the experimental study of the interplay of superfluidity and DW order.