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Phys Rev Lett. 2009 Jun 12;102(23):230402. Epub 2009 Jun 8.

Observation of Fermi polarons in a tunable Fermi liquid of ultracold atoms.

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Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.


We have observed Fermi polarons, dressed spin-down impurities in a spin-up Fermi sea of ultracold atoms. The polaron manifests itself as a narrow peak in the impurities' rf spectrum that emerges from a broad incoherent background. We determine the polaron energy and the quasiparticle residue for various interaction strengths around a Feshbach resonance. At a critical interaction, we observe the transition from polaronic to molecular binding. Here, the imbalanced Fermi liquid undergoes a phase transition into a Bose liquid, coexisting with a Fermi sea.

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