Lattice Boltzmann simulations of a strongly interacting two-dimensional Fermi gas

Brewer, Jasmine; Mendoza, M.; Young, Ryan E.; Romatschke, Paul

doi:10.1103/physreva.93.013618

Cited by 22 publications

(20 citation statements)

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“…Furthermore, for trapped Fermi gases it is known that non-hydrodynamic collective modes are featured in descriptions beyond Navier-Stokes, but lie outside the regime of validity of these descriptions for strong interactions [19,23]. If the interactions are weak, and quasiparticle degrees of freedom are well defined, kinetic theory also predicts the presence of non-hydrodynamic modes, which are purely damped excitations (10).…”

Section: P0mentioning

confidence: 99%

“…(9) contain one non-hydrodynamic mode for both the quadrupole mode and breathing mode in D = 3 (cf. [19,22]). In the strong coupling limit τ R → 0, these non-hydrodynamic modes obey the dispersion relations (8) upon setting τ π = τ R .…”

Section: P0mentioning

confidence: 99%

“…For kinetic theory with a simple collision term in the form of a relaxation time approximation, the collective modes of a Fermi gas in a harmonic trap have been analyzed before [3,[19][20][21]. For a relaxation time τ R in the collision term of the Boltzmann equation, one finds dispersion relations for the breathing and quadrupole modes given by…”

Section: P0mentioning

confidence: 99%

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Nonhydrodynamic Transport in Trapped Unitary Fermi Gases

Brewer

Romatschke

2015

Phys. Rev. Lett.

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Many strongly coupled fluids are known to share similar hydrodynamic transport properties. In this work we argue that this similarity could extend beyond hydrodynamics to transient dynamics through the presence of non-hydrodynamic modes. We review non-hydrodynamic modes in kinetic theory and gauge/gravity duality and discuss their signatures in trapped Fermi gases close to unitarity. Reanalyzing previously published experimental data, we find hints of non-hydrodynamic modes in cold Fermi gases in two and three dimensions.In the past decade, precision experiments of ultracold quantum gases at unitarity have enabled the study of transport phenomena in strongly coupled systems [1][2][3][4][5][6]. Usually, transport phenomena are encoded in hydrodynamic transport coefficients, such as the speed of sound, shear and bulk viscosities, heat conductivities, spin diffusion coefficients, etc. Presently, the speed of sound has been measured in a three dimensional unitary Fermi gas [2,6], and the shear viscosity has been constrained for both three dimensional and two dimensional Fermi gases [4,5]. The extremely low values of shear viscosity (when expressed in units of entropy) in particular suggest similarities between cold Fermi gases close to unitarity and very different systems such as hot quark gluon plasmas [7,8], high-temperature superconductors [9] and strongly coupled fluids described by black holes via the AdS/CFT conjecture [10], all of which share similar transport behavior. This apparent similarity in otherwise completely different physical systems suggests that these systems could be part of a broader class of so-called strongly interacting quantum fluids (SIQFs).It is conceivable that SIQFs share other properties besides their similar (hydrodynamic) transport behavior. This could be important because it could imply that it is possible to learn about one example of SIQFs (say high-temperature superconductors) through studying a different SIQF for which a particular trait is more easily accessible. In the present study we will investigate ultracold Fermi gases close to unitarity and argue that they exhibit properties similar to black hole SIQFs.One property that is quite remarkable about black hole SIQFs is that they do not seem to possess a description in terms of weakly coupled quasiparticles [11]. Instead, black holes can be characterized in terms of their ringdown spectrum, similar to a glass struck (lightly) with a fork [12]. Some of these quasinormal modes can be recognized to be the well-known hydrodynamic modes, i.e. sound and shear excitations. Others do not have an equivalent in (Navier-Stokes) hydrodynamics, and are thus non-hydrodynamic, but nevertheless affect transport properties (particularly on short time scales).If properties of SIQFs are universal, one would expect the presence of non-hydrodynamic modes in cold Fermi gases close to unitarity. This provides the motivation for searching for non-hydrodynamic modes in cold Fermi gases, both theoretically and experimentally.Transport in hydrodynamics...

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Section: P0mentioning

confidence: 99%

Section: P0mentioning

confidence: 99%

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Nonhydrodynamic Transport in Trapped Unitary Fermi Gases

Brewer

Romatschke

2015

Phys. Rev. Lett.

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“…One may argue that most of these fluids are quantal, hence not captured by classical lattice kinetic models, for instance, the LB schemes discussed in this work. To this regard, it is worth pointing out that the lattice Boltzmann method has been already used to study two-dimensional Fermi gas at unitarity [21], providing a procedure to calculate transport coefficients precisely. Furthermore, the LB versions for quantum wave functions are available since long, and they are based on exactly the same stream-collide paradigm, if only with a suitable scattering matrix for collisions [19].…”

Section: Discussionmentioning

confidence: 99%

Short-Lived Lattice Quasiparticles for Strongly Interacting Fluids

Jimenez

Succi

2015

Entropy

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It is shown that lattice kinetic theory based on short-lived quasiparticles proves very effective in simulating the complex dynamics of strongly interacting fluids (SIF). In particular, it is pointed out that the shear viscosity of lattice fluids is the sum of two contributions, one due to the usual interactions between particles (collision viscosity) and the other due to the interaction with the discrete lattice (propagation viscosity). Since the latter is negative, the sum may turn out to be orders of magnitude smaller than each of the two contributions separately, thus providing a mechanism to access SIF regimes at ordinary values of the collisional viscosity. This concept, as applied to quantum superfluids in one-dimensional optical lattices, is shown to reproduce shear viscosities consistent with the AdS-CFT holographic bound on the viscosity/entropy ratio. This shows that lattice kinetic theory continues to hold for strongly coupled hydrodynamic regimes where continuum kinetic theory may no longer be applicable.

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“…where n is the number density of particles (we let  = = k 1 B throughout). The effects of a realistic non-ideal equation of state on collective mode behavior in a viscous fluid typically require numerical treatments such as those presented in [23].…”

Section: Assumptionsmentioning

confidence: 99%

Higher-harmonic collective modes in a trapped gas from second-order hydrodynamics

Lewis

Romatschke

2017

New J. Phys.

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Utilizing a second-order hydrodynamics formalism, the dispersion relations for the frequencies and damping rates of collective oscillations as well as spatial structure of these modes up to the decapole oscillation in both two-and three-dimensional gas geometries are calculated. In addition to higherorder modes, the formalism also gives rise to purely damped 'non-hydrodynamic' modes. We calculate the amplitude of the various modes for both symmetric and asymmetric trap quenches, finding excellent agreement with an exact quantum mechanical calculation. We find that higher-order hydrodynamic modes are more sensitive to the value of shear viscosity, which may be of interest for the precision extraction of transport coefficients in Fermi gas systems.

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Lattice Boltzmann simulations of a strongly interacting two-dimensional Fermi gas

Cited by 22 publications

References 50 publications

Nonhydrodynamic Transport in Trapped Unitary Fermi Gases

Nonhydrodynamic Transport in Trapped Unitary Fermi Gases

Short-Lived Lattice Quasiparticles for Strongly Interacting Fluids

Higher-harmonic collective modes in a trapped gas from second-order hydrodynamics

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