Tunneling transport in strongly-interacting atomic Fermi gases

Strongly-interacting ultracold atomic Fermi gases are a formidable playground for benchmarking many-body theories of fermionic matter and to explore novel states of matter. In particular, the capability of tuning the interparticle interactions by means of Feshbach resonances allows for the exploration of different superfluid regimes, from a Bardeen-Cooper-Schrieffer (BCS) gas of Cooper pairs to a Bose-Einstein condensate (BEC) of tightly bound molecules, realizing the celebrated BEC-BCS crossover. Close to resonance, interactions are so strong to reach the unitary limit, where the system displays universal properties common to nuclear matter and neutron stars. The theoretical modelling of those atomic gases is challenged by strong correlations between the particles. The experimental investigation provides thus a valuable support in their understanding. In particular, transport measurements represent a powerful and versatile tool, connecting microscopic properties to macroscopic measurable quantities. In this thesis, we experimentally investigate tunneling transport of strongly-interacting Fermi gases of lithium- 6 atoms through a thin optical barrier. The importance of tunneling transport measurements is twofold. On the one hand, they unveil the coherence properties of the many-body system, through the celebrated Josephson effect, arising when two condensed reservoirs are weakly connected by a thin repulsive barrier. On the other hand, they provide fundamental insights on the role of the excitations in the conduction dynamics, allowing for a direct comparison with ordinary solid-state systems. In particular, during my thesis work, we realize a current- biased Josephson junction between fermionic superfluids by exploiting the high spatial resolution of our apparatus, and the dynamic control over repulsive optical potentials provided by a spatial light modulator. In a first experiment, we characterize the junction conductance for different superfluid regimes [1]. We observe dc Josephson effect by measuring a highly non-linear current-chemical potential characteristic, the analogous of the current-voltage of a superconducting Josephson junction, and the typical sinusoidal current-phase relation. We measure a dissipationless supercurrent up to a critical value, the critical current, that we map as a function of the barrier properties and of the interaction strength. By comparing our results with an analytic model, we extract the order parameter of fermionic superfluids, namely their condensed fraction, the quantitative determination of which has been so far indirect and somewhat inconclusive. In a second experiment, we characterize the operation of our junction across the superfluid transition [2]. As temperature increases, we observe a transition from highly non-linear to Ohmic current-chemical potential curve, which signals the breakdown of the Josephson effect. From the behavior of the maximum Josephson supercurrent, we extract a lower limit for the critical temperature of the superfluid transition. Remarkably, we observe the condensate to feed not only the Josephson supercurrent, but also the resistive one. In stark contrast with superconducting junctions, we detect indeed a large anomalous normal conductance at low temperature, arising from the coherent coupling between the condensate and phononic Bogoliubov-Anderson excitations. Furthermore, we measure a large conductance even above the critical temperature, due to the hydrodynamic behavior of unpaired fermions at unitarity. In the last part of my thesis work, we exploit the versatility of our experimental system to investigate another aspect of superfluidity: the vortex nucleation dynamics in the wake of a moving obstacle. In particular, we measure the critical velocity for vortex shedding in oblate quasi-homogeneous ultracold Fermi gases, providing a preliminary step towards the exploration of quantum turbulence in strongly-interacting fermionic superfluids. [1] W. J. Kwon, et al., Science, 369, 6499 (2020). [2] G. Del Pace, et al., Phys. Rev. Lett. 126.5, p.055301 (2021).