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Observation of a pairing pseudogap in a two-dimensional Fermi gas

Abstract

Pairing of fermions is ubiquitous in nature, underlying many phenomena. Examples include superconductivity, superfluidity of 3He, the anomalous rotation of neutron stars, and the crossover between Bose–Einstein condensation of dimers and the BCS (Bardeen, Cooper and Schrieffer) regime in strongly interacting Fermi gases. When confined to two dimensions, interacting many-body systems show even more subtle effects1, many of which are not understood at a fundamental level. Most striking is the (as yet unexplained) phenomenon of high-temperature superconductivity in copper oxides, which is intimately related to the two-dimensional geometry of the crystal structure. In particular, it is not understood how the many-body pairing is established at high temperature, and whether it precedes superconductivity. Here we report the observation of a many-body pairing gap above the superfluid transition temperature in a harmonically trapped, two-dimensional atomic Fermi gas in the regime of strong coupling. Our measurements of the spectral function of the gas are performed using momentum-resolved photoemission spectroscopy2,3, analogous to angle-resolved photoemission spectroscopy in the solid state4. Our observations mark a significant step in the emulation of layered two-dimensional strongly correlated superconductors using ultracold atomic gases.

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Figure 1: Measuring the spectral function.
Figure 2: Pairing in the strongly interacting two-dimensional Fermi gas.
Figure 3: Interaction dependence of the quasiparticle dispersion.
Figure 4: Temperature dependence of the quasiparticle dispersion.

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Acknowledgements

We thank A. Georges, C. Kollath, D. Pertot, D. Petrov, M. Randeria, W. Zwerger and M. Zwierlein for discussions. The work was supported by EPSRC (EP/G029547/1), Daimler-Benz Foundation (B.F.), Studienstiftung and DAAD (M.F.).

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The experimental set-up was devised and constructed by M.F., B.F., E.V. and M. Köhl, data-taking was performed by M.F., B.F., E.V. and M. Koschorreck, data analysis was performed by M.F., B.F. and M. Koschorreck, numerical modelling was performed by B.F., and the manuscript was written by M. Köhl with contributions from all co-authors.

Corresponding author

Correspondence to Michael Köhl.

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The authors declare no competing financial interests.

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Feld, M., Fröhlich, B., Vogt, E. et al. Observation of a pairing pseudogap in a two-dimensional Fermi gas. Nature 480, 75–78 (2011). https://doi.org/10.1038/nature10627

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