Emergence of Sound in a Tunable Fermi Fluid

Since its discovery in 1957, Landau's Fermi liquid theory has been extensively applied across diverse systems such as liquid Helium-3, normal metals, and Fermi polarons in ultracold Fermi gases. One of the central themes in Landau's Fermi liquid theory is the propagation of sound. In this study, we investigate the emergence of first sound in a uniform Fermi gas of 6 Li atoms with tunable interaction strength. By measuring the density response of the gas to an oscillating linear potential, we observe a pronounced peak emerge in the response amplitude spectrum as the interaction increases. We measure the sound velocity and the attenuation as a function of the interaction strength and the temperature of the gas. Additionally, we examine the quasiparticle excitations by directly measuring the momentum space tomography.