Quantum degenerate Fermi gases provide a remarkable opportunity to study strongly interacting fermions. In contrast to other Fermi systems, such as superconductors, neutron stars or the quark-gluon plasma of the early Universe, these gases have low densities and their interactions can be precisely controlled over an enormous range. Previous experiments with Fermi gases have revealed condensation of fermion pairs. Although these and other studies were consistent with predictions assuming superfluidity, proof of superfluid behaviour has been elusive. Here we report observations of vortex lattices in a strongly interacting, rotating Fermi gas that provide definitive evidence for superfluidity. The interaction and therefore the pairing strength between two 6Li fermions near a Feshbach resonance can be controlled by an external magnetic field. This allows us to explore the crossover from a Bose–Einstein condensate of molecules to a Bardeen–Cooper–Schrieffer superfluid of loosely bound pairs. The crossover is associated with a new form of superfluidity that may provide insights into high-transition-temperature superconductors.
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We thank P. Zarth for experimental assistance and C. Stan for contributions in the early stages of the experiment. We also acknowledge discussions with the participants of the OCTS conference in Ohio, and thank J. Anglin, Z. Hadzibabic, D. Kleppner and A. Leanhardt for a critical reading of the manuscript. This work was supported by the NSF, ONR, ARO and NASA.
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
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Zwierlein, M., Abo-Shaeer, J., Schirotzek, A. et al. Vortices and superfluidity in a strongly interacting Fermi gas. Nature 435, 1047–1051 (2005). https://doi.org/10.1038/nature03858
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