Universal spin transport in a strongly interacting Fermi gas

Abstract

Transport of fermions is central in many fields of physics. Electron transport runs modern technology, defining states of matter such as superconductors and insulators, and electron spin, rather than charge, is being explored as a new carrier of information [1]. Neutrino transport energizes supernova explosions following the collapse of a dying star [2], and hydrodynamic transport of the quark-gluon plasma governed the expansion of the early Universe [3]. However, our understanding of non-equilibrium dynamics in such strongly interacting fermionic matter is still limited. Ultracold gases of fermionic atoms realize a pristine model for such systems and can be studied in real time with the precision of atomic physics [4,5]. It has been established that even above the superfluid transition such gases flow as an almost perfect fluid with very low viscosity [3,6] when interactions are tuned to a scattering resonance. However, here we show that spin currents, as opposed to mass currents, are maximally damped, and that interactions can be strong enough to reverse spin currents, with opposite spin components reflecting off each other. We determine the spin drag coefficient, the spin diffusivity, and the spin susceptibility, as a function of temperature on resonance and show that they obey universal laws at high temperatures. At low temperatures, the spin diffusivity approaches a minimum value set by /m, the quantum limit of diffusion, where is the reduced Planck's constant and m the atomic mass. For repulsive interactions, our measurements appear to exclude a metastable ferromagnetic state [7][8][9].Understanding the transport of spin, as opposed to the transport of charge, is of high interest for the novel field of spintronics [1]. While charge currents are unaffected by electron-electron scattering due to momentum conservation, spin currents will intrinsically damp due to collisions between opposite spin electrons, as their relative momentum is not conserved. This phenomenon is known as spin drag [10,11]. It is expected to contribute significantly to the damping of spin currents in doped semiconductors [12]. The random collision events also lead to spin diffusion, the tendency for spin currents to flow such as to even out spatial gradients in the spin density, which has been studied in high-temperature superconductors [13] and in liquid 3 He-4 He solutions [14,15]. Creating spin currents poses a major challenge in electronic systems where mobile spins are scattered by their environment and by each other. However, in ultracold atoms we have the freedom to first prepare an essentially non-interacting spin mixture, separate atoms spatially via magnetic field gradients, and only then induce strong interactions. Past observations of spin currents in ultracold Fermi gases [16,17] were made in the weaklyinteracting regime. Here we access the regime near a Feshbach resonance [5], where interactions are as strong as allowed by quantum mechanics (the unitarity limit). We measure spin transport properties, the spin drag coeffici...

Bouncing motion and penetration dynamics in multicomponent Bose-Einstein condensates

“…In contrast, in the case of a relatively strongly immiscible system, binary BECs continue to bounce. Such repetitive bouncing motion between atoms has been observed only in a Tonks-Girardeau gas [19], a Fermi gas [20], and matter-wave solitons [21]. In addition, numerical simulations of the Gross-Pitaevskii (GP) equation suggest that the penetrability and penetration time between BECs can be tuned by slightly changing the atomic interaction strength.…”

Spin-Seebeck effect in a strongly interacting Fermi gas

“…3(a), includes corrections due to spin-heat coupling, but they are negligibly small, so that one can safely take σ s = nτ s /m with τ s being the spindrag relaxation time [8] measured in Ref. [6] and calculated in Ref. [9].…”

“…Spin transport in a strongly interacting, two-component Fermi gas was investigated experimentally in Ref. [6]. It is the purpose of this article to study the associated heat transport, i.e., thermo-spin effects, in a similar setting.…”

Realizing exactly solvableSU(N)magnets with thermal atoms

“…Motivated by the exploration of how quantum systems evolve after quantum quenches and whether (or how) they equilibrate and/or thermalize [46], especially in the presence of long-range interactions [6,7], we first study spin diffusion [44,47,48] in a system of g atoms only. Due to the crucial use of representation-theoretic techniques, our calculations not only are exponentially faster than naive exact diagonalization but also, for N = 2, yield a closed-form expression for all n. We then present a protocol that employs both g and e states to create Greenberger-Horne-Zeilinger (GHZ) states [49], which could be used to approach the Heisenberg limit for metrology and clock precision [50].…”

“…Spin diffusion is the process by which evolution under a generic spin Hamiltonian causes initially ordered states to diffuse [44,47,48]. We take the initial state |ψ(0) = |1 ⊗m 1 |2 ⊗m 2 .…”