Saturation of Current Dissipation in a Strongly-Interacting Fermi-Hubbard System

Transport dissipation rates measured through diffusion, momentum relaxation, or conductivity are key to understanding a system's transport mechanisms. Highly correlated materials, such as high-TC cuprates, reveal unconventional scalings and saturation levels of transport dissipation rates, suggesting violations of Fermi-liquid theory and the breakdown of quasiparticle descriptions of transport. We report on direct measurements of the interaction-dominated AC conductivity of fermionic potassium-40 in a 3D optical lattice. We measure the conductivity spectrum of a mass current excited by applying an oscillatory force to the system. As the system is phonon- and defect-free, the dominant source of current dissipation is two-body s-wave collisions between fermionic charge carriers. We tune the interaction strength via a Feshbach resonance, and observe two distinct regimes in the current dissipation rate: at low interaction strengths, the dissipation rate increases linearly with the on-site collision rate; at sufficiently high interaction strengths, the dissipation rate saturates and is limited by the tunnelling rate, evoking the notion of "lattice unitarity." We compare measurements to a Boltzmann transport calculation of the 3D Fermi-Hubbard model. These measurements can provide insights into unconventional "bad metals"; future directions include investigating transport in excited bands, highly-correlated insulating states, and superfluids.