Abstract

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.

1. Department of Physics, MIT-Harvard Center for Ultracold Atoms, and Research Laboratory of Electronics, MIT, Cambridge, Massachusetts 02139, USA
2. †Present address: Lawrence Berkeley National Laboratory, One Cyclotron Road, MS 88R0192, Berkeley, California 94720, USA

Correspondence to: M. W. Zwierlein¹ Correspondence and requests for materials should be addressed to M.W.Z. (Email: zwierlei@mit.edu).

Received 9 May 2005; Accepted 31 May 2005

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