Determination of the Fermion Pair Size in a Resonantly Interacting Superfluid

Abstract

Fermionic superfluidity requires the formation of pairs. The actual size of these fermion pairs varies by orders of magnitude from the femtometer scale in neutron stars and nuclei to the micrometer range in conventional superconductors. Many properties of the superfluid depend on the pair size relative to the interparticle spacing. This is expressed in BCS-BEC crossover theories, describing the crossover from a Bardeen-Cooper-Schrieffer (BCS) type superfluid of loosely bound and large Cooper pairs to Bose-Einstein condensation (BEC) of tightly bound molecules. Such a crossover superfluid has been realized in ultracold atomic gases where high temperature superfluidity has been observed. The microscopic properties of the fermion pairs can be probed with radio-frequency (rf) spectroscopy. Previous work was difficult to interpret due to strong and not well understood final state interactions. Here we realize a new superfluid spin mixture where such interactions have negligible influence and present fermion-pair dissociation spectra that reveal the underlying pairing correlations. This allows us to determine the spectroscopic pair size in the resonantly interacting gas to be 2.6(2)/kF (kF is the Fermi wave number). The pairs are therefore smaller than the interparticle spacing and the smallest pairs observed in fermionic superfluids. This finding highlights the importance of small fermion pairs for superfluidity at high critical temperatures. We have also identified transitions from fermion pairs into bound molecular states and into many-body bound states in the case of strong final state interactions.

8 pages, 7 figures; Figures updated; New Figures added; Updated discussion of fit functions

References in corpus (6)

• Vortices and Superfluidity in a Strongly Interacting Fermi Gas
• Observation of the Pairing Gap in a Strongly Interacting Fermi Gas
• Ultracold fermions and the SU(N) Hubbard model
• Radio-Frequency Transitions on Weakly-Bound Ultracold Molecules
• Competition between final-state and pairing-gap effects in the radio-frequency spectra of ultracold Fermi atoms
• The Condition for Universality at Resonance and Direct Measurement of Pair Wavefunctions Using rf Spectroscopy

Cited by in corpus (4)

• Using photoemission spectroscopy to probe a strongly interacting Fermi gas
• Collisional stability of a three-component degenerate Fermi gas
• Spectral signatures of the Fulde-Ferrell-Larkin-Ovchinnikov order parameter in one-dimensional optical lattices
• Radio frequency spectroscopy of a strongly imbalanced Feshbach-resonant Fermi gas