Virial Theorem and Universality in a Unitary Fermi Gas

TL;DR

This paper demonstrates that a unitary Fermi gas of lithium-6 obeys the virial theorem across various temperatures, confirming its universal behavior and enabling a simple thermometry method based on volume measurements.

Contribution

It provides experimental verification of the virial theorem and universality in a unitary Fermi gas, and introduces a new volume thermometry technique.

Findings

Obeys virial theorem over wide temperature range

Breathing mode frequency aligns with universal hydrodynamics

Supports universal thermodynamic behavior in unitary gases

Abstract

Unitary Fermi gases, where the scattering length is large compared to the interparticle spacing, can have universal properties, which are independent of the details of the interparticle interactions when the range of the scattering potential is negligible. We prepare an optically-trapped, unitary Fermi gas of $^6$Li, tuned just above the center of a broad Feshbach resonance. In agreement with the universal hypothesis, we observe that this strongly-interacting many-body system obeys the virial theorem for an ideal gas over a wide range of temperatures. Based on this result, we suggest a simple volume thermometry method for unitary gases. We also show that the observed breathing mode frequency, which is close to the unitary hydrodynamic value over a wide range of temperature, is consistent with a universal hydrodynamic gas with nearly isentropic dynamics.