Universal spin transport in a strongly interacting Fermi gas

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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 information1. Neutrino transport energizes supernova explosions following the collapse of a dying star2, and hydrodynamic transport of the quark–gluon plasma governed the expansion of the early Universe3. 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 physics4. Even above the superfluid transition, such gases flow as an almost perfect fluid with very low viscosity when interactions are tuned to a scattering resonance3,5,6,7,8. In this hydrodynamic regime, collective density excitations are weakly damped6,7. Here we experimentally investigate spin excitations in a Fermi gas of 6Li 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 /m, the quantum limit of diffusion, where /m is Planck’s constant divided by 2π and m the atomic mass. For repulsive interactions, our measurements seem to exclude a metastable ferromagnetic state9,10,11.

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Figure 1: Observation of spin current reversal in a resonant collision between two oppositely spin-polarized clouds of fermions.
Figure 2: Spin drag coefficient of a trapped Fermi gas with resonant interactions.
Figure 3: Spin diffusivity of a trapped Fermi gas.
Figure 4: Spin susceptibility on resonance.

References

  1. 1

    Wolf, S. A. Spintronics: a spin-based electronics vision for the future. Science 294, 1488–1495 (2001)

  2. 2

    Burrows, A. Neutrinos from supernova explosions. Annu. Rev. Nucl. Part. Sci. 40, 181–212 (1990)

  3. 3

    Schäfer, T. & Teaney, D. Nearly perfect fluidity: from cold atomic gases to hot quark gluon plasmas. Rep. Prog. Phys. 72, 126001 (2009)

  4. 4

    Inguscio, M., Ketterle, W., Salomon, C. (eds) Ultracold Fermi Gases (Proc. Int. School of Physics ‘Enrico Fermi’, Course CLXIV, IOS, 2008)

  5. 5

    O'Hara, K. M., Hemmer, S. L., Gehm, M. E., Granade, S. R. & Thomas, J. E. Observation of a strongly interacting degenerate Fermi gas of atoms. Science 298, 2179–2182 (2002)

  6. 6

    Riedl, S. et al. Collective oscillations of a Fermi gas in the unitarity limit: temperature effects and the role of pair correlations. Phys. Rev. A 78, 053609 (2008)

  7. 7

    Cao, C. et al. Universal quantum viscosity in a unitary Fermi gas. Science 331, 58–61 (2011)

  8. 8

    Enss, T., Haussmann, R. & Zwerger, W. Viscosity and scale invariance in the unitary Fermi gas. Ann. Phys. 326, 770–796 (2011)

  9. 9

    Jo, G.-B. et al. Itinerant ferromagnetism in a Fermi gas of ultracold atoms. Science 325, 1521–1524 (2009)

  10. 10

    Stringari, S. Density and spin response function of a normal Fermi gas at unitarity. Phys. Rev. Lett. 102, 110406 (2009)

  11. 11

    Duine, R. A., Polini, M., Stoof, H. T. C. & Vignale, G. Spin drag in an ultracold Fermi gas on the verge of ferromagnetic instability. Phys. Rev. Lett. 104, 220403 (2010)

  12. 12

    D'Amico, I. & Vignale, G. Theory of spin Coulomb drag in spin-polarized transport. Phys. Rev. B 62, 4853–4857 (2000)

  13. 13

    Weber, C. P. et al. Observation of spin Coulomb drag in a two-dimensional electron gas. Nature 437, 1330–1333 (2005)

  14. 14

    D’Amico, I. & Vignale, G. Coulomb interaction effects in spin-polarized transport. Phys. Rev. B 65, 085109 (2002)

  15. 15

    Gedik, N., Orenstein, J., Liang, R., Bonn, D. A. & Hardy, W. N. Diffusion of nonequilibrium quasi-particles in a cuprate superconductor. Science 300, 1410–1412 (2003)

  16. 16

    Garwin, R. L. & Reich, H. A. Self-diffusion and nuclear relaxation in He3 . Phys. Rev. 115, 1478–1492 (1959)

  17. 17

    Anderson, A. C., Edwards, D. O., Roach, W. R., Sarwinski, R. E. & Wheatley, J. C. Thermal and magnetic properties of dilute solutions of He3 in He4 at low temperatures. Phys. Rev. Lett. 17, 367–372 (1966)

  18. 18

    DeMarco, B. & Jin, D. S. Spin excitations in a Fermi gas of atoms. Phys. Rev. Lett. 88, 040405 (2002)

  19. 19

    Du, X., Luo, L., Clancy, B. & Thomas, J. E. Observation of anomalous spin segregation in a trapped Fermi gas. Phys. Rev. Lett. 101, 150401 (2008)

  20. 20

    Zwierlein, M. W., Schirotzek, A., Schunck, C. H. & Ketterle, W. Fermionic superfluidity with imbalanced spin populations. Science 311, 492–496 (2006)

  21. 21

    Shin, Y., Zwierlein, M., Schunck, C., Schirotzek, A. & Ketterle, W. Observation of phase separation in a strongly interacting imbalanced Fermi gas. Phys. Rev. Lett. 97, 030401 (2006)

  22. 22

    Vichi, L. & Stringari, S. Collective oscillations of an interacting trapped Fermi gas. Phys. Rev. A 60, 4734–4737 (1999)

  23. 23

    Nascimbène, S., Navon, N., Jiang, K. J., Chevy, F. & Salomon, C. Exploring the thermodynamics of a universal Fermi gas. Nature 463, 1057–1060 (2010)

  24. 24

    Bruun, G. M., Recati, A., Petchick, C. J., Smith, H. & Stringari, S. Collisional properties of a polarized Fermi gas with resonant interactions. Phys. Rev. Lett. 100, 240406 (2008)

  25. 25

    Bruun, G. M. Spin diffusion in Fermi gases. New J. Phys. 13, 035005 (2011)

  26. 26

    Polini, M. & Vignale, G. Spin drag and spin-charge separation in cold Fermi gases. Phys. Rev. Lett. 98, 266403 (2007)

  27. 27

    Parish, M. M. & Huse, D. A. Evaporative depolarization and spin transport in a unitary trapped Fermi gas. Phys. Rev. A 80, 063605 (2009)

  28. 28

    Gaebler, J. P. et al. Observation of pseudogap behavior in a strongly interacting Fermi gas. Nature Phys. 6, 569–573 (2010)

  29. 29

    Perali, A. et al. Evolution of the normal state of a strongly interacting Fermi gas from a pseudogap phase to a molecular Bose gas. Phys. Rev. Lett. 106, 060402 (2011)

  30. 30

    Partridge, G. B., Li, W., Kamar, R. I., Liao, Y. & Hulet, R. G. Pairing and phase separation in a polarized Fermi gas. Science 311, 503–505 (2006)

  31. 31

    Nascimbène, S. et al. Collective oscillations of an imbalanced Fermi gas: axial compression modes and polaron effective mass. Phys. Rev. Lett. 103, 170402 (2009)

  32. 32

    Ku, M. et al. Equation of state of a strongly interacting atomic Fermi gas. Bull. Am. Phys. Soc. 55, abstr. W6. 00001 (2010); available at 〈http://meetings.aps.org/link/BAPS.2010.DAMOP.W6.1〉 (2010)

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Acknowledgements

We thank G. Bruun, C. Pethick, D. Huse, 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.

Author information

All authors contributed to the experimental work. A.S. analysed the data. M.K. developed the algorithm for thermometry. M.W.Z. performed theoretical calculations. A.S. and M.W.Z wrote the manuscript.

Correspondence to Ariel Sommer.

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The authors declare no competing financial interests.

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This file contains a Supplementary Discussion, Supplementary Figures 1-2 with legends, Supplementary Methods, Supplementary Equations and additional references. (PDF 270 kb)

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Sommer, A., Ku, M., Roati, G. et al. Universal spin transport in a strongly interacting Fermi gas. Nature 472, 201–204 (2011) doi:10.1038/nature09989

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