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Using photoemission spectroscopy to probe a strongly interacting Fermi gas


Ultracold atomic gases provide model systems in which to study many-body quantum physics. Recent experiments using Fermi gases have demonstrated a phase transition to a superfluid state with strong interparticle interactions1,2,3,4,5,6. This system provides a realization of the ‘BCS–BEC crossover’7 connecting the physics of Bardeen–Cooper–Schrieffer (BCS) superconductivity with that of Bose–Einstein condensates (BECs). Although many aspects of this system have been investigated, it has not yet been possible to measure the single-particle excitation spectrum (a fundamental property directly predicted by many-body theories). Here we use photoemission spectroscopy to directly probe the elementary excitations and energy dispersion in a strongly interacting Fermi gas of 40K atoms. In the experiments, a radio-frequency photon ejects an atom from the strongly interacting system by means of a spin-flip transition to a weakly interacting state. We measure the occupied density of single-particle states at the cusp of the BCS–BEC crossover and on the BEC side of the crossover, and compare these results to that for a nearly ideal Fermi gas. We show that, near the critical temperature, the single-particle spectral function is dramatically altered in a way that is consistent with a large pairing gap. Our results probe the many-body physics in a way that could be compared to data for the high-transition-temperature superconductors. As in photoemission spectroscopy for electronic materials, our measurement technique for ultracold atomic gases directly probes low-energy excitations and thus can reveal excitation gaps and/or pseudogaps. Furthermore, this technique can provide an analogue of angle-resolved photoemission spectroscopy for probing anisotropic systems, such as atoms in optical lattice potentials.

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Figure 1: Photoemission spectroscopy for ultracold atom gases.
Figure 2: Extracting the three-dimensional momentum distribution.
Figure 3: Single-particle excitation spectra obtained using photoemission spectroscopy of ultracold atoms.
Figure 4: Energy distribution curves for a strongly interacting Fermi gas.
Figure 5: The occupied density of single-particle states.


  1. Regal, C. A., Greiner, M. & Jin, D. S. Observation of resonance condensation of fermionic atom pairs. Phys. Rev. Lett. 92, 040403 (2004)

    Article  ADS  CAS  Google Scholar 

  2. Chin, C. et al. Observation of the pairing gap in a strongly interacting Fermi gas. Science 305, 1128–1130 (2004)

    Article  ADS  CAS  Google Scholar 

  3. Partridge, G. B., Strecker, K. E., Kamar, R. I., Jack, M. W. & Hulet, R. G. Molecular probe of pairing in the BEC-BCS crossover. Phys. Rev. Lett. 95, 020404 (2005)

    Article  ADS  CAS  Google Scholar 

  4. Zwierlein, M. W., Abo-Shaeer, J. R., Schirotzek, A., Schunck, C. H. & Ketterle, W. Vortices and superfluidity in a strongly interacting Fermi gas. Nature 435, 1047–1051 (2005)

    Article  ADS  CAS  Google Scholar 

  5. Luo, L., Clancy, B., Joseph, J., Kinast, J. & Thomas, J. E. Measurement of the entropy and critical temperature of a strongly interacting Fermi gas. Phys. Rev. Lett. 98, 080402 (2007)

    Article  ADS  CAS  Google Scholar 

  6. Tarruell, L. et al. in Ultra-Cold Fermi Gases (Proc. Internat. School Phys. ‘Enrico Fermi’, Course 164) (eds Inguscio, M., Ketterle, W. & Salomon, C.) 845–855 (IOS Press, 2008)

    Google Scholar 

  7. Giorgini, S., Pitaevskii, L. P. & Stringari, S. Theory of ultracold Fermi gases. Rev. Mod. Phys. (in the press)

  8. Damascelli, A. Probing the electronic structure of complex systems by ARPES. Phys. Scr. T109, 61–74 (2004)

    Article  ADS  CAS  Google Scholar 

  9. Regal, C. A. & Jin, D. S. Measurement of positive and negative scattering lengths in a Fermi gas of atoms. Phys. Rev. Lett. 90, 230404 (2003)

    Article  ADS  CAS  Google Scholar 

  10. Regal, C. A., Ticknor, C., Bohn, J. L. & Jin, D. S. Creation of ultracold molecules from a Fermi gas of atoms. Nature 424, 47–50 (2003)

    Article  ADS  CAS  Google Scholar 

  11. Gupta, S. et al. Radiofrequency spectroscopy of ultracold fermions. Science 300, 1723–1726 (2003)

    Article  ADS  CAS  Google Scholar 

  12. Schunck, C. H., Shin, Y., Schirotzek, A., Zwierlein, M. W. & Ketterle, W. Pairing without superfluidity: The ground state of an imbalanced Fermi mixture. Science 316, 867–870 (2007)

    Article  ADS  CAS  Google Scholar 

  13. Schunck, C. H., Shin, Y., Schirotzek, A. & Ketterle, W. Determination of the fermion pair size in a resonantly interacting superfluid. Preprint at 〈〉 (2008)

  14. Dao, T.-L., Georges, A., Dalibard, J., Salomon, C. & Carusotto, I. Measuring the one-particle excitations of ultracold fermionic atoms by stimulated Raman spectroscopy. Phys. Rev. Lett. 98, 240402 (2007)

    Article  ADS  Google Scholar 

  15. Chin, C. & Julienne, P. S. Radio-frequency transitions on weakly bound ultracold molecules. Phys. Rev. A 71, 012713 (2005)

    Article  ADS  Google Scholar 

  16. Yu, Z. & Baym, G. Spin-correlation functions in ultracold paired atomic-fermion systems: Sum rules, self-consistent approximations, and mean fields. Phys. Rev. A 73, 063601 (2006)

    Article  ADS  Google Scholar 

  17. Punk, M. & Zwerger, W. Theory of rf-spectroscopy of strongly interacting fermions. Phys. Rev. Lett. 99, 170404 (2007)

    Article  ADS  CAS  Google Scholar 

  18. Perali, A. & Strinati, G. C. Competition between final-state and pairing gap effects in the radio-frequency spectra of ultracold Fermi atoms. Phys. Rev. Lett. 100, 010402 (2008)

    Article  ADS  CAS  Google Scholar 

  19. Basu, S. & Mueller, E. J. Final-state effects in the radio frequency spectrum of strongly interacting fermions. Preprint at 〈〉 (2007)

  20. Veillette, M. et al. Radio frequency spectroscopy of a strongly imbalanced Feshbach-resonant Fermi gas. Preprint at 〈〉 (2008)

  21. He, Y., Chien, C.-C., Chen, Q. & Levin, K. Temperature and final state effects in radio frequency spectroscopy experiments on atomic Fermi gases. Preprint at 〈〉 (2008)

  22. Randeria, M. in Models and Phenomenology for Conventional and High-Temperature Superconductivity (Proc. Internat. School Phys. ‘Enrico Fermi’, Course 136) (eds Iadonisi, G., Schrieffer, J. R. & Chiofalo, M. L.) 53–57 (IOS Press, 1998)

    Google Scholar 

  23. Janko, B., Maly, J. & Levin, K. Pseudogap effects induced by resonant pair scattering. Phys. Rev. B 56, R11407–R11410 (1997)

    Article  ADS  CAS  Google Scholar 

  24. Yanase, Y. & Yamada, K. Theory of pseudogap phenomena in high-T c cuprates based on the strong coupling superconductivity. J. Phys. Soc. Jpn 68, 2999–3015 (1999)

    Article  ADS  CAS  Google Scholar 

  25. Perali, A., Pieri, A., Strinati, G. C. & Castellani, C. Pseudogap and spectral function from superconducting fluctuations to the bosonic limit. Phys. Rev. B 66, 024510 (2002)

    Article  ADS  Google Scholar 

  26. Bruun, G. M. & Baym, G. Bragg spectroscopy of cold atomic Fermi gases. Phys. Rev. A 74, 033623 (2006)

    Article  ADS  Google Scholar 

  27. Bulgac, A., Drut, J. E., Magierski, P. & Wlazlowski, G. Gap and pseudogap of a unitary Fermi gas by quantum Monte Carlo. Preprint at 〈〉 (2008)

  28. Barnea, N. Superfluid to insulator phase transition in a unitary Fermi gas. Preprint at 〈〉 (2008)

  29. Stewart, J. T., Gaebler, J. P., Regal, C. A. & Jin, D. S. Potential energy of a 40K Fermi gas in the BCS-BEC crossover. Phys. Rev. Lett. 97, 220406 (2006)

    Article  ADS  CAS  Google Scholar 

  30. Gaebler, J. P., Stewart, J. T., Bohn, J. L. & Jin, D. S. p-wave Feshbach molecules. Phys. Rev. Lett. 98, 200403 (2007)

    Article  ADS  CAS  Google Scholar 

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We acknowledge funding from the US NSF. We thank E. Cornell, D. Dessau and the JILA BEC group for discussions.

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Correspondence to D. S. Jin.

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Stewart, J., Gaebler, J. & Jin, D. Using photoemission spectroscopy to probe a strongly interacting Fermi gas. Nature 454, 744–747 (2008).

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