Nonlinear spin diffusion and spin rotation in a trapped Fermi gas

Enss, Tilman

doi:10.1103/physreva.91.023614

Cited by 8 publications

(19 citation statements)

References 37 publications

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“…was confirmed experimentally (85). For ultracold fermions at strong interaction, the temperature and polarization dependence of the transverse and longitudinal diffusivities has been computed within kinetic theory using the many-body T matrix which incorporates medium scattering (59,76). This makes it an interesting proposition to measure D 0 and D ⊥ 0 in ultracold Fermi gases, if one can reach low enough temperatures, and compare to theoretical predictions.…”

Section: 22mentioning

confidence: 91%

Universal Spin Transport and Quantum Bounds for Unitary Fermions

Enss

Thywissen

2019

Annu. Rev. Condens. Matter Phys.

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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 quasi-particle 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. 1 arXiv:1805.05354v1 [cond-mat.quant-gas]

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Section: 22mentioning

confidence: 91%

Universal Spin Transport and Quantum Bounds for Unitary Fermions

Enss

Thywissen

2019

Annu. Rev. Condens. Matter Phys.

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“…At strong interaction when the mean free path ∼ 1µm is much smaller than the Thomas-Fermi length and the typical minimal spin-helix pitch ∼ 5µm, we expect that the trap averaged transport coefficients are close to the homogeneous values. In this regime the dynamics are essentially local [30].…”

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confidence: 99%

“…The lines on Fig. 2(a) show a kinetic theory both with and without medium scattering (solid and dashed lines, respectively) calculated in the |M | → 1 limit [11,30]. The model also accounts for inhomogeneities in the following way: first, the collision integral is solved to compute the transverse spin diffusion time and LR parameter for a 2D homogeneous system with the same spin density and temperature as the trap center [11,35].…”

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confidence: 99%

Observation of Quantum-Limited Spin Transport in Strongly Interacting Two-Dimensional Fermi Gases

et al. 2017

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We measure the transport properties of two-dimensional ultracold Fermi gases during transverse demagnetization in a magnetic field gradient. Using a phase-coherent spin-echo sequence, we are able to distinguish bare spin diffusion from the Leggett-Rice effect, in which demagnetization is slowed by the precession of spin current around the local magnetization. When the two-dimensional scattering length is tuned to be comparable to the inverse Fermi wave vector k −1 F , we find that the bare transverse spin diffusivity reaches a minimum of 1.7(6) /m, where m is the bare particle mass. The rate of demagnetization is also reflected in the growth rate of the s-wave contact, observed using time-resolved spectroscopy. At unitarity, the contact rises to 0.28(3)k 2 F per particle, measuring the breaking of scaling symmetry. Our observations support the conjecture that in systems with strong scattering, the local relaxation rate is bounded from above by kBT / .Conjectured quantum bounds on transport appear to be respected and nearly saturated by quark-gluon plasmas [1, 2], unitary Fermi gases [3][4][5][6][7][8][9][10][11], and bad metals [12,13]. For many modalities of transport these bounds can be recast as an upper bound on the rate of local relaxation to equilibrium 1/τ r k B T / , where k B is the Boltzmann constant and T is temperature [14,15]. Systems that saturate this "Planckian" bound do not have well defined quasiparticles promoting transport [1,[12][13][14][15]. A canonical example is the quantum critical regime, where one expects diffusivity D ∼ /m, a ratio of shear viscosity to entropy density η/s ∼ /k B , and a conductivity that is linear in T [4, 12, 13]. These limiting behaviors can be understood by combining τ r with a propagation speed v ∼ k B T /m, for example D ∼ v 2 τ r . This argument applies to ultracold three-dimensional (3D) Fermi gases, whose behavior in the strongly interacting regime is controlled by the quantum critical point at divergent scattering length, zero temperature, and zero density [4,16,17]. In such systems, one observes D 2 /m [6-8] and η/s 0.4 /k B [3], compatible with conjectured quantum bounds.However in attractive two-dimensional (2D) Fermi gases, scale invariance is broken by the finite bound-state pair size, so the strongly interacting regime is no longer controlled by a quantum critical point [16,[18][19][20][21][22][23]. Strikingly, an extreme violation of the conjectured D /m bound has been observed in an ultracold 2D Fermi gas: a spin diffusivity of 6.3(8) × 10 −3 /m near ln(k F a 2D ) = 0 [24], where k F is the Fermi momentum and a 2D is the 2D s-wave scattering length. No similarly dramatic effect of dimensionality is observed in charge conductivity [12] or bulk viscosity [25], and such a low spin diffusivity is unexplained by theory [11,19].In this work, we recreate the conditions of Ref. [24], and study the demagnetization dynamics of ultracold 2D Fermi gases using both a coherent spin-echo sequence [8] and time-resolved spectroscopy [7]. We find a modification of th...

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“…Moreover, despite the local character of the interactions, collective phenomena were observed, including demagnetization and transverse spinwaves in the former, and a time-dependent separation (segregation) of the spin densities and spin self-rephasing in the latter. Although mean-field and kinetic theory formulations have explained some of these phenomena [8][9][10][11][12][13][14][15][16][17][18], a theory capable of describing all the time scales and the interplay between spin, motion, and interactions has not been developed.In this work, we develop a framework that accounts for the coupling of spin and motion in weakly interacting Fermi gases. We qualitatively reproduce and explain all phenomena of the aforementioned experiments and obtain quantitative agreement with the results of Ref.…”

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Dynamics of Interacting Fermions in Spin-Dependent Potentials

Koller¹,

Wall²,

Mundinger³

et al. 2016

Phys. Rev. Lett.

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Recent experiments with dilute trapped Fermi gases observed that weak interactions can drastically modify spin transport dynamics and give rise to robust collective effects including global demagnetization, macroscopic spin waves, spin segregation, and spin self-rephasing. In this work we develop a framework for studying the dynamics of weakly interacting fermionic gases following a spin-dependent change of the trapping potential which illuminates the interplay between spin, motion, Fermi statistics, and interactions. The key idea is the projection of the state of the system onto a set of lattice spin models defined on the single-particle mode space. Collective phenomena, including the global spreading of quantum correlations in real space, arise as a consequence of the long-ranged character of the spin model couplings. This approach achieves good agreement with prior measurements and suggests a number of directions for future experiments.The interplay between spin and motional degrees of freedom in interacting electron systems has been a longstanding research topic in condensed matter physics. Interactions can modify the behavior of individual electrons and give rise to emergent collective phenomena such as superconductivity and colossal magnetoresistance [1]. Theoretical understanding of non-equilibrium dynamics in interacting fermionic matter is limited, however, and many open questions remain. Ultracold atomic Fermi gases, with precisely controllable parameters, offer an outstanding opportunity to investigate the emergence of collective behavior in out-of-equilibrium settings.Progress in this direction has been made in recent experiments with ultracold spin-1/2 fermionic vapors, where initially spin-polarized gases were subjected to a spin-dependent trapping potential ( Fig. 1) implemented by a magnetic field gradient [2-4], or a spin-dependent harmonic trapping frequency [5-8] -equivalent to a spatially-varying gradient. Even in the weakly interacting regime, drastic modifications of the single-particle dynamics were reported. Moreover, despite the local character of the interactions, collective phenomena were observed, including demagnetization and transverse spinwaves in the former, and a time-dependent separation (segregation) of the spin densities and spin self-rephasing in the latter. Although mean-field and kinetic theory formulations have explained some of these phenomena [8][9][10][11][12][13][14][15][16][17][18], a theory capable of describing all the time scales and the interplay between spin, motion, and interactions has not been developed.In this work, we develop a framework that accounts for the coupling of spin and motion in weakly interacting Fermi gases. We qualitatively reproduce and explain all phenomena of the aforementioned experiments and obtain quantitative agreement with the results of Ref. [7]. In this formulation the state of the system is represented as a superposition of spin configurations which live on lattices whose sites correspond to modes of the underlying single-particle sy...

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Nonlinear spin diffusion and spin rotation in a trapped Fermi gas

Cited by 8 publications

References 37 publications

Universal Spin Transport and Quantum Bounds for Unitary Fermions

Universal Spin Transport and Quantum Bounds for Unitary Fermions

Observation of Quantum-Limited Spin Transport in Strongly Interacting Two-Dimensional Fermi Gases

Dynamics of Interacting Fermions in Spin-Dependent Potentials

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