This paper presents a theoretical investigation of various viscous relaxation mechanisms in unitary Fermi gases. The authors find that the relaxation time of the system in the pseudogap regime has a universal temperature dependence associated with anomalous transport phenomena of different strongly correlated systems. They further construct the relation between this universality of relaxation time and the anomalously small shear viscosity.
This paper presents a theoretical investigation of various viscous relaxation mechanisms in unitary Fermi gases. The authors find that the relaxation time of the system in the pseudogap regime has a universal temperature dependence associated with anomalous transport phenomena of different strongly correlated systems. They further construct the relation between this universality of relaxation time and the anomalously small shear viscosity.
We theoretically investigate the shear viscosity η in the BCS-BEC crossover regime of an ultracold Fermi gas with a Feshbach resonance. Within the framework of the strong-coupling self-consistent T-matrix approximation, we examine how a strong pairing interaction associated with a Feshbach resonance affects this transport coefficient, in the normal state above the superfluid phase transition temperature Tc. We show that, while η diverges in both the weak-coupling BCS and strong-coupling BEC limits, it becomes small in the unitary regime. The minimum of η is obtained, not at the unitarity, but slightly in the strong-coupling BEC side. This deviation is consistent with the recent experiment on a 6Li Fermi gas. In the weak-coupling BCS regime, we also find that η exhibits anomalous temperature dependence near Tc, which is deeply related to the pseudogap phenomenon originating form strong pairing fluctuations.
The shear viscosity has been an important topic in ultracold Fermi gases, and it has served as a diagnostic of various theories. Due to the complicated phase structures of population-imbalanced (polarized) Fermi gases with tunable attraction, past works on the shear viscosity mainly focused on unpolarized Fermi gases. Here we investigate the shear viscosity of homogeneous, population-imbalanced Fermi superfluid at finite temperatures by a pairing fluctuation theory for thermodynamical quantities and a gauge-invariant linear response theory for transport coefficients. The Cooper pairs lead to the anomalous shear viscosity analogous to the shear viscosity. We derive an exact relation connecting certain thermodynamic quantities and transport coefficients at the mean-field level for polarized unitary Fermi superfluids. An approximate relation beyond mean-field is proposed and only exhibits mild deviations from our numerical results. In the unitary and Bose-Einstein condensation (BEC) regimes, the total shear viscosity increases with the polarization because the excess majority fermions cause gapless excitations acting like a normal fluid. Moreover, competition among the excess fermions, noncondensed pairs, and fermionic quasiparticles may lead to non-monotonic behavior of the ratio between the shear viscosity and relaxation time as the polarization increases.
We measure universal temperature-independent density shifts for the thermal conductivity κ T and shear viscosity η , relative to the high temperature limits, for a normal phase unitary Fermi gas confined in a box potential. We show that a time-dependent kinetic theory model enables extraction of the hydrodynamic transport times τ η and τ κ from the time-dependent free decay of a spatially periodic density perturbation, yielding the static transport properties and density shifts, corrected for finite relaxation times. Published by the American Physical Society 2024
The shear viscosity has been shown to be equal to the product of the pressure and relaxation time in normal scale-invariant fluids, but the presence of superfluidity at low temperatures can alter the relation. Using the mean-field BCS-Leggett theory with a gauge-invariant linear response theory for unitary Fermi superfluids, we present an explicit relation between thermodynamic quantities, including the pressure and chemical potential, and transport coefficients, including the shear viscosity, superfluid density, and anomalous shear viscosity from momentum transfer via Cooper pairs. The relation is modified when pairing fluctuations associated with noncondensed Cooper pairs are considered. Within a pairing fluctuation theory consistent with the BCS-Leggett ground state, we found an approximate relation for unitary Fermi superfluids. The exact mean-field relation and the approximate one with pairing fluctuations advance our understanding of relations between equilibrium and transport quantities in superfluids, and they help determine or constrain quantities which otherwise can be difficult to measure.
Transport in strongly correlated fermions cannot be understood by fermionic quasiparticles alone. We present a theoretical framework for quantum transport that incorporates strong local correlations of fermion pairs. These contact correlations add essential contributions to viscous, thermal and sound transport coefficients. The bulk viscosity, in particular, receives its dominant contribution from pair excitations. Moreover, it can be measured elegantly by observing the response to a time-dependent scattering length even when the fluid is not moving. Rapid changes of the scattering length drive the system far out of local equilibrium, and we show how it relaxes back to equilibrium following a hydrodynamic attractor before a Navier–Stokes description becomes valid. This paper summarizes a talk given at the Symposium “Open questions in the quantum many-body problem” at the Institut Henry Poincaré, Paris, in July 2024.
We review recent advances in experimental and theoretical understanding of spin transport in strongly interacting Fermi gases. The central new phenomenon is the observation of a lower bound on the (bare) spin diffusivity in the strongly interacting regime. Transport bounds are of broad interest for the condensed matter community, with a conceptual similarity to observed bounds in shear viscosity and charge conductivity. We discuss the formalism of spin hydrodynamics, how dynamics are parameterized by transport coefficients, the effect of confinement, the role of scale invariance, the quasiparticle picture, and quantum critical transport. We conclude by highlighting open questions, such as precise theoretical bounds, relevance to other phases of matter, and extensions to lattice systems.