Emergence of Sound in a Tunable Fermi Fluid

TL;DR

This paper experimentally investigates a tunable Fermi fluid to understand the microscopic basis of Landau's Fermi-liquid theory, observing sound emergence, quasiparticle behavior, and deviations at strong interactions, thus providing a versatile platform for studying Fermi liquids.

Contribution

It provides the first measurement of the Lindhard function in a dilute atomic Fermi gas and demonstrates the emergence of sound and quasiparticle excitations from first principles.

Findings

Observation of the Lindhard function in an ideal Fermi gas

Emergence of sound in the transition from collisionless to hydrodynamic regime

Direct visualization of quasiparticle excitations and their evolution

Abstract

Landau's Fermi-liquid (FL) theory has been successful at the phenomenological description of the normal phase of many different Fermi systems. Using a dilute atomic Fermi fluid with tunable interactions, we investigate the microscopic basis of Landau's theory with a system describable from first principles. We study transport properties of an interacting Fermi gas by measuring its density response to a periodic external perturbation. In an ideal Fermi gas, we measure for the first time the celebrated Lindhard function. As the system is brought from the collisionless to the hydrodynamic regime, we observe the emergence of sound, and find that the experimental observations are quantitatively understood with a first-principle transport equation for the FL. When the system is more strongly interacting, we find deviations from such predictions. Finally, we observe the shape of the quasiparticle excitations directly from momentum-space tomography and see how it evolves from the collisionless to the collisional regime. Our study establishes this system as a clean platform for studying Landau's theory of the FL and paves the way for extending the theory to more exotic conditions, such as nonlinear dynamics and FLs with strong correlations in versatile settings.