Universal spin transport in a strongly interacting Fermi gas

Year: 2011

Authors: Sommer A., Ku M., Roati G., Zwierlein M.W.

Autors Affiliation: Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA;
MIT-Harvard Center for Ultracold Atoms, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA;
Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA;
CNR – Istituto Nazionale di Ottica, University of Florence,50019 Sesto Fiorentino, Italy;
LENS, University of Florence, 50019 Sesto Fiorentino, Italy

Abstract: 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(3,5-8). 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 (h) over bar /m, the quantum limit of diffusion, where (h) over bar is Planck’s constant divided by 2 pi and m the atomic mass. For repulsive interactions, our measurements seem to exclude a metastable ferromagnetic state(9-11).

Journal/Review: NATURE

Volume: 472 (7342)      Pages from: 201  to: 204

More Information: We thank G. Bruun, C. Pethick, D. H use, R. Duine and W. Zwerger for discussions, and A. Schirotzek for help with the early stages of the experiment. This work was supported by the NSF, AFOSR-MURI, ARO-MURI, ONR, DARPA YFA, a grant from the Army Research Office with funding from the DARPA OLE programme, the David and Lucille Packard Foundation and the Alfred P. Sloan Foundation.
KeyWords: Coulomb Drag; Diffusion; Atoms
DOI: 10.1038/nature09989

Citations: 236
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