Fermi-pressure-assisted cavity superradiance in a mesoscopic Fermi gas

TL;DR

This paper investigates how Fermi pressure influences superradiance in a mesoscopic Fermi gas within an optical cavity, revealing a non-monotonic threshold behavior and a spin-density-wave phase, advancing understanding of light-matter interactions in few-fermion systems.

Contribution

It demonstrates the role of Fermi pressure in superradiant phase transitions and introduces a regime with spin-density-wave order in a mesoscopic Fermi gas.

Findings

Superradiant threshold varies non-monotonically with density.

Minimum threshold occurs when Fermi and recoil wavevectors are comparable.

Observation of spin-density-wave phase due to opposite light-induced forces.

Abstract

We study the superradiant phase transition of a mesoscopic Fermi gas comprising between a few tens and a few thousand $^6$Li atoms in a high-finesse cavity across a wide range of densities. We observe a non-monotonic variation of the superradiant threshold as a function of density, with a minimum reached when the Fermi and recoil wavevectors are comparable. The minimum corresponds to a crossover between Fermi pressure-assisted ordering and Pauli blocking of photon scattering, in good agreement with theory. This interpretation is confirmed by a study of the atom-number dependence of the ordering threshold and photon number scaling. Lastly, we demonstrate the operation of our mesoscopic system in a regime where light-induced forces are opposite for the two spin components, leading to an ordered phase with a spin-density-wave character. Our system opens the perspective of studying few-fermion systems with strong and coherent light-matter coupling.