Observation of conformal symmetry breaking and scale invariance in expanding Fermi gases

TL;DR

This study investigates the hydrodynamic behavior of expanding Fermi gases, confirming scale invariance at resonance and observing symmetry breaking away from resonance, with implications for understanding quantum many-body systems.

Contribution

It provides precise experimental evidence of conformal symmetry breaking and scale invariance in expanding Fermi gases across different interaction strengths.

Findings

Scale-invariant hydrodynamics observed at resonance.

Bulk viscosity consistent with zero at resonance.

Conformal symmetry breaking evident away from resonance.

Abstract

We precisely test scale invariance and local thermal equilibrium in the hydrodynamic expansion of a Fermi gas of atoms as a function of interaction strength. After release from an anisotropic optical trap, we observe that a resonantly interacting gas obeys scale-invariant hydrodynamics, where the mean square cloud size $\langle{\mathbf{r}}^2\rangle=\langle x^2+y^2+z^2\rangle$ expands ballistically (like a noninteracting gas) and the energy-averaged bulk viscosity is consistent with zero, $0.005(0.016)\,\hbar\,n$, with $n$ the density. In contrast, the aspect ratios of the cloud exhibit anisotropic "elliptic" flow with an energy-dependent shear viscosity. Tuning away from resonance, we observe conformal symmetry breaking, where $\langle{\mathbf{r}}^2\rangle$ deviates from ballistic flow.