Measurement of dynamical correlations in a Fermi gas driven about unitarity

The dynamical bulk viscosity ξ, which determines dissipation in expansion and compression, vanishes for a scale-invariant fluid such as the unitary Fermi gas. However, the product a²ξ, where a is the s-wave scattering length, has been predicted to be well defined at and near unitarity. Recent theoretical work has shown that a²ξ also governs the response to temporal modulation of a. Here, we present measurements of two observables in a driven unitary fermionic system – dissipative heating and temporal relaxation. 

 

We prepare a degenerate Fermi gas of potassium-40 tuned to unitarity by a Feshbach resonance. Rapid modulation of the s-wave scattering length is achieved by driving currents through the wires of a chip in close proximity to the atoms. The resulting out-of-phase response of the contact parameter manifests as a dissipative heating rate, which we measure by extracting the energy of the gas. In a linear response framework, we observe a heating rate that scales quadratically with the amplitude of the drive and place bounds on the magnitude of the dynamical bulk viscosity. At sufficiently slow oscillation frequencies, we probe the temporal response of two-body correlations directly through time-resolved radiofrequency spectroscopy. Our observations pave the way towards future measurements of hydrodynamic attractors in periodically modulated systems.