Figure - Observation of spin current reversal in a resonant collision between two oppositely spin-polarized clouds of fermions. (a) shows the total column density and (b) the difference in column densities of the two clouds (red: spin up, blue: spin down), in 1 ms intervals during the first 20 ms after the magnetic field is set to the Feshbach resonance at 834 G. The collision leads to the formation of a high-density interface between the two spin states. (c) Time evolution of the separation between the centers of mass of the two spin states. Even after half a second, there is still substantial spin separation. The diffusion time indicates a diffusivity on the order of h/m, where m is the atomic mass. (d) Shows the harmonic trapping potential along the axis of symmetry.
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 is being explored as a new carrier of information. Neutrino transport energizes supernova explosions following the collapse of a dying star, and hydrodynamic transport of the quark-gluon plasma governed the expansion of the early Universe. 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. Even above the superfluid transition such gases flow as an almost perfect fluid with very low viscosity, when interactions are tuned to a scattering resonance. In this hydrodynamic regime, collective excitations are weakly damped.
Here we show, in turn, that spin excitations are maximally damped. A spin current is induced in a Fermi gas by spatially separating the two spin components and observing their evolution in an external trapping potential. We demonstrate that interactions can be strong enough to reverse spin currents, with opposite spin components reflecting off each other. Near equilibrium we obtain 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. Near the Fermi temperature, the spin diffusivity approaches a minimum value set by h/m, the quantum limit of diffusion, where h is Planck's constant and m the atomic mass.
Furthermore, we determine the spin susceptibility in this strongly interacting Fermi gas as a function of temperature and show that it obeys the Curie law at high temperatures. At low temperatures, the spin susceptibility no longer matches the compressibility, a behavior indicative of a Fermi liquid.