Ultracold Fermi gases with tunable interactions provide a test bed for exploring the many-body physics of strongly interacting quantum systems1,2,3,4. Over the past decade, experiments have investigated many intriguing phenomena, and precise measurements of ground-state properties have provided benchmarks for the development of theoretical descriptions. Metastable states in Fermi gases with strong repulsive interactions5,6,7,8,9,10,11 represent an exciting area of development. The realization of such systems is challenging, because a strong repulsive interaction in an atomic quantum gas implies the existence of a weakly bound molecular state, which makes the system intrinsically unstable against decay. Here we use radio-frequency spectroscopy to measure the complete excitation spectrum of fermionic 40K impurities resonantly interacting with a Fermi sea of 6Li atoms. In particular, we show that a well-defined quasiparticle exists for strongly repulsive interactions. We measure the energy and the lifetime of this ‘repulsive polaron’9,12,13, and probe its coherence properties by measuring the quasiparticle residue. The results are well described by a theoretical approach that takes into account the finite effective range of the interaction in our system. We find that when the effective range is of the order of the interparticle spacing, there is a substantial increase in the lifetime of the quasiparticles. The existence of such a long-lived, metastable many-body state offers intriguing prospects for the creation of exotic quantum phases in ultracold, repulsively interacting Fermi gases.
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Bloch, I., Dalibard, J. & Zwerger, W. Many-body physics with ultracold gases. Rev. Mod. Phys. 80, 885–964 (2008)
Giorgini, S., Pitaevskii, L. P. & Stringari, S. Theory of ultracold atomic Fermi gases. Rev. Mod. Phys. 80, 1215–1274 (2008)
Radzihovsky, L. & Sheehy, D. E. Imbalanced Feshbach-resonant Fermi gases. Rep. Prog. Phys. 73, 076501 (2010)
Chevy, F. & Mora, C. Ultra-cold polarized Fermi gases. Rep. Prog. Phys. 73, 112401 (2010)
Duine, R. A. & MacDonald, A. H. Itinerant ferromagnetism in an ultracold atom Fermi gas. Phys. Rev. Lett. 95, 230403 (2005)
LeBlanc, L. J., Thywissen, J. H., Burkov, A. A. & Paramekanti, A. Repulsive Fermi gas in a harmonic trap: ferromagnetism and spin textures. Phys. Rev. A 80, 013607 (2009)
Conduit, G. J., Green, A. G. & Simons, B. D. Inhomogeneous phase formation on the border of itinerant ferromagnetism. Phys. Rev. Lett. 103, 207201 (2009)
Jo, G.-B. et al. Itinerant ferromagnetism in a Fermi gas of ultracold atoms. Science 325, 1521–1524 (2009)
Pilati, S., Bertaina, G., Giorgini, S. & Troyer, M. Itinerant ferromagnetism of a repulsive atomic Fermi gas: a quantum Monte Carlo study. Phys. Rev. Lett. 105, 030405 (2010)
Chang, S.-Y., Randeria, M. & Trivedi, N. Ferromagnetism in the upper branch of the Feshbach resonance and the hard-sphere Fermi gas. Proc. Natl Acad. Sci. USA 108, 51–54 (2011)
Sanner, C. et al. Correlations and pair formation in a repulsively interacting Fermi gas. Preprint at 〈http://arxiv.org/abs/1108.2017〉 (2012)
Massignan, P. & Bruun, G. M. Repulsive polarons and itinerant ferromagnetism in strongly polarized Fermi gases. Eur. Phys. J. D 65, 83–89 (2011)
Schmidt, R. & Enss, T. Excitation spectra and rf response near the polaron-to-molecule transition from the functional renormalization group. Phys. Rev. A 83, 063620 (2011)
Landau, L. D. The theory of a Fermi liquid. Sov. Phys. JETP 3, 920–925 (1957)
Lobo, C., Recati, A., Giorgini, S. & Stringari, S. Normal state of a polarized Fermi gas at unitarity. Phys. Rev. Lett. 97, 200403 (2006)
Schirotzek, A., Wu, C.-H., Sommer, A. & Zwierlein, M. W. Observation of Fermi polarons in a tunable Fermi liquid of ultracold atoms. Phys. Rev. Lett. 102, 230402 (2009)
Navon, N., Nascimbène, S., Chevy, F. & Salomon, C. The equation of state of a low-temperature Fermi gas with tunable interactions. Science 328, 729–732 (2010)
Nascimbène, S. et al. Fermi-liquid behavior of the normal phase of a strongly interacting gas of cold atoms. Phys. Rev. Lett. 106, 215303 (2011)
Pekker, D. et al. Competition between pairing and ferromagnetic instabilities in ultracold Fermi gases near Feshbach resonances. Phys. Rev. Lett. 106, 050402 (2011)
Chin, C., Grimm, R., Julienne, P. S. & Tiesinga, E. Feshbach resonances in ultracold gases. Rev. Mod. Phys. 82, 1225–1286 (2010)
Naik, D. et al. Feshbach resonances in the 6Li-40K Fermi-Fermi mixture: elastic versus inelastic interactions. Eur. Phys. J. D 65, 55–65 (2011)
Petrov, D. S. Three-boson problem near a narrow Feshbach resonance. Phys. Rev. Lett. 93, 143201 (2004)
Punk, M., Dumitrescu, P. T. & Zwerger, W. Polaron-to-molecule transition in a strongly imbalanced Fermi gas. Phys. Rev. A 80, 053605 (2009)
Combescot, R., Recati, A., Lobo, C. & Chevy, F. Normal state of highly polarized Fermi gases: simple many-body approaches. Phys. Rev. Lett. 98, 180402 (2007)
Sadeghzadeh, K., Bruun, G. M., Lobo, C., Massignan, P. & Recati, A. Metastability in spin-polarized Fermi gases and quasiparticle decays. N. J. Phys. 13, 055011 (2011)
Nascimbène, S. et al. Collective oscillations of an imbalanced Fermi gas: axial compression modes and polaron effective mass. Phys. Rev. Lett. 103, 170402 (2009)
Prokof’ev, N. & Svistunov, B. Fermi-polaron problem: diagrammatic Monte Carlo method for divergent sign-alternating series. Phys. Rev. B 77, 020408 (2008)
Chin, C. et al. Observation of the pairing gap in a strongly interacting Fermi gas. Science 305, 1128–1130 (2004)
Shin, Y., Schunck, C. H., Schirotzek, A. & Ketterle, W. Tomographic rf spectroscopy of a trapped Fermi gas at unitarity. Phys. Rev. Lett. 99, 090403 (2007)
Stewart, J. T., Gaebler, J. P. & Jin, D. S. Using photoemission spectroscopy to probe a strongly interacting Fermi gas. Nature 454, 744–747 (2008)
Spiegelhalder, F. M. et al. All-optical production of a degenerate mixture of 6Li and 40K and creation of heteronuclear molecules. Phys. Rev. A 81, 043637 (2010)
Trenkwalder, A. et al. Hydrodynamic expansion of a strongly interacting Fermi-Fermi mixture. Phys. Rev. Lett. 106, 115304 (2011)
Gezerlis, A., Gandolfi, S., Schmidt, K. E. & Carlson, J. Heavy-light fermion mixtures at unitarity. Phys. Rev. Lett. 103, 060403 (2009)
Kasevich, M. & Chu, S. Laser-cooling below a photon recoil with three-level atoms. Phys. Rev. Lett. 69, 1741–1744 (1992)
We thank A. Sidorov for contributions in the early stage of the experiments, and T. Enss, S. Giorgini, W. Ketterle, J. Levinsen, C. Lobo, D. Petrov, A. Recati, R. Schmidt, J. Song, C. Trefzger, P. Zoller, W. Zwerger, M. Zwierlein and, in particular, M. Baranov for many discussions. We acknowledge support from the Austrian Science Fund FWF through the SFB FoQuS. M.Z. is supported by the Lise Meitner programme of the FWF. P.M. is indebted to M. Lewenstein for support through the ERC Advanced Grant QUAGATUA.
The authors declare no competing financial interests.
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Kohstall, C., Zaccanti, M., Jag, M. et al. Metastability and coherence of repulsive polarons in a strongly interacting Fermi mixture. Nature 485, 615–618 (2012). https://doi.org/10.1038/nature11065
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