Universal particle and entropy transport in strongly interacting Fermi gases far from equilibrium

Abstract

Transport experiments are among the most important methods to probe and characterize strongly interacting fermionic systems and the myriad states of matter that exist therein. The majority of existing experimental and theoretical studies in this context have focused on bulk systems in the hydrodynamic limit. In this regime, the transport properties of these systems can be described by linear response coefficients such as viscosity and conductivity which are fundamentally properties of the system in a state of thermal equilibrium. This thesis presents experiments probing the transport properties of strongly interacting gases of fermionic 6Li in a regime far from thermal equilibrium. Using the experimental toolbox of ultracold atomic gases, we shape the system into two reservoirs connected by a tunable, ballistic channel and measure currents of particles, entropy, and spin flowing through it. When the reservoirs are superfluid, the particle and entropy currents exhibit a highly nonlinear response to biases between the reservoirs in chemical potential and temperature. This observation reveals that the gas in the channel is perturbed by the biases to a state far from equilibrium. Furthermore, by inducing particle loss localized in the channel, we controllably break detailed balance. This principle underlies the transport properties of thermal systems, so its violation adds another dimension to the far-from-equilibrium nature of our system. In this setting, we observe several phenomena with no known microscopic explanation and therefore pose fundamental questions regarding transport in strongly interacting Fermi systems far from equilibrium. In a first set of experiments, we study coupled particle and entropy transport between unitary superfluid reservoirs across the 1D-2D crossover of the channel. Our central observation is a superfluid-enhanced Peltier effect: nonlinear particle currents advectively transport entropy along with them, and the average entropy transported per particle --- the Seebeck coefficient --- is robust against changes in the channel's geometry. The system also exhibits a diffusive mode of entropy transport, whose strong suppression in 1D channels results in a long-lived non-equilibrium steady state --- a signature of the breakdown of the Wiedemann-Franz law. We directly measure the entropy produced by these irreversible flows. In the absence of a microscopic or hydrodynamic theory, we develop a nonlinear phenomenological model within the formalism of generalized gradient dynamics to codify our observations and characterize the system's transport properties with a similarly compact framework as in the linear response regime. In a second set of experiments, we investigate the origin of the Seebeck coefficient's robustness by exploring a large parameter space spanned by global properties of the reservoirs and local properties of the channel. The former are the interaction strength and degeneracy that characterize the universal thermodynamics of the BCS-BEC crossover and normal-superfluid phase transition, and the latter are the transverse confinement and potential in the channel. The Seebeck coefficient depends only on the global thermodynamic properties and not on the details of the channel, suggesting that this far-from-equilibrium property has its origin in the universal equilibrium properties of the reservoirs. In contrast, the magnitudes of advective and diffusive currents vary significantly with both the channel and reservoir properties, revealing a change in the nature of the excitations responsible for transport across the BCS-BEC crossover. The phenomenological model describes the system over the entire parameter range explored here, smoothly connecting regimes of linear and nonlinear response. In a final set of experiments, we induce particle loss in the channel via photon scattering by atoms or Feshbach molecules and study its influence on transport with unitary superfluid reservoirs. Both types of dissipation have the same qualitative effects: particle currents respond more linearly and are reduced in magnitude, but remain larger than the values corresponding to a dissipation-free Fermi liquid, while entropy and spin diffusion are enhanced to levels consistent with a Fermi liquid. Effective microscopic theories based on multiple Andreev reflections quantitatively reproduce our observations of particle transport but fail for entropy and spin transport. We observe that, unlike the geometric properties of the channel, dissipation changes the advectively transported entropy per particle and appears to violate Onsager reciprocity, as evidenced by a decoupling of the Seebeck and Peltier coefficients of the phenomenological model.