Universal spin transport in a strongly interacting Fermi gas

Atomic spin currents in Fermi gases Strongly interacting Fermi gases are ubiquitous in nature, from electrons in high-temperature superconductors, to nuclear matter. Their transport properties, such as the speed of diffusion, are poorly understood. Sommer et al . use controlled collisions of ultracold atomic clouds to investigate spin transport in a strongly interacting Fermi gas. They find that the spin excitations are maximally damped (leading to high spin drag), and that interactions are strong enough to reverse spin currents so that opposite spin components reflect off each other. The speed of diffusion is set by a fundamental quantum limit. The results have implications for any area involving fermion transport, from spintronics to studies of the early Universe. Transport of fermions, particles with half-integer spin, is central to 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 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 . 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 3 , 5 , 6 , 7 , 8 . In this hydrodynamic regime, collective density excitations are weakly damped 6 , 7 . Here we experimentally investigate spin excitations in a Fermi gas of 6 Li atoms, finding that, in contrast, they are maximally damped. A spin current is induced by spatially separating 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 components of opposite spin 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. In the degenerate regime, the spin diffusivity approaches a value set by ℏ/ m , the quantum limit of