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Strongly Interacting Fermi Gases: Birth, Life, and Fate of Quasiparticles

By Rudolf Grimm, University of Innsbruck

An impurity immersed in a Fermi sea represents an elementary building block for quantum many-body physics. The behavior of the resulting quasiparticles (Fermi polarons) lies at the heart of interesting phases of quantum matter, with broad interest ranging from strongly interacting Fermi gases in the normal phase and high-Tc superconductors to Kondo physics. A polaron’s life strongly depends on the conditions of its interactions with its environment and exhibits a variety of intriguing phenomena.

Our model system is a small sample of 40K atoms immersed in a large Fermi sea of 6Li atoms[1]. We control the interaction via an interspecies Feshbach resonance, with the additional possibility of extremely fast optical control. An interferometric detection scheme in the time domain[2] enables us to unveil the full story of the polaron’s life. The birth is observed on a short time scale of a few microseconds[3], directly set by the Fermi time. The polaron then lives with a well-defined quasiparticle energy, but undergoes a loss of coherence by thermal excitations[4]. A new chapter beyond single-impurity physics is opened up by interpolaronic interactions mediated by the Fermi sea[3].

We briefly discuss further new developments, like bosonic 41K impurities in the 6Li Fermi sea, and the creation of a novel Fermi-Fermi mixture of dysprosium and potassium atoms.

  1. C. Kohstall, M. Zaccanti, M. Jag, A. Trenkwalder, P. Massignan, G. Bruun, R. Grimm, Nature 485, 615 (2012).
  2. M. Knap, A. Shashi, Y. Nishida, A. Imambekov, D. A. Abanin, E. Demler, Phys. Rev. X S. 2, 041020 (2012).
  3. M. Cetina, M. Jag, R. S. Lous, I. Fritsche, J. T. M. Walraven, R. Grimm, J. Levinsen, M. M. Parish, R. Schmidt, M. Knap, E. Demler, Ultrafast many-body interferometry of impurities coupled to a Fermi sea, to be published.
  4. M. Cetina, M. Jag, R. S. Lous, J. T. M. Walraven, R. Grimm, R. Christensen, G. Bruun, Phys. Rev. Lett. 115, 135302 (2015).