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. In the June 23 issue of Nature, we report our observation 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.
|The pictures show vortex lattices on the BEC-side of the Feshbach resonance (left), in the unitary regime on resonance (middle) and on the BCS-side of the resonance (right).|
|A condensate of Fermion pairs (red) is trapped in the waist of a focussed Laser beam (pink). Two additional Laser beams (green) rotate around the edges to stir the condensate. Current-carrying coils (blue) generate the magnetic field used for axial confinement and to tune the interaction strength by means of a Feshbach resonance. After releasing the atomic cloud from the electromagnetic trap, the cloud expands ballistically and inverts its aspect ratio. Resonant absorption imaging yields a density profile of the atomic cloud containing vortices.|
Sodium vs Lithium
Vortices in Gases: Shown is a Vortex pattern in bosonic Sodium atoms (green cartoon) in a magnetic trap, Vortices in tightly bound Lithium molecules (red-blue cartoon) and a vortex lattice in loosely bound Fermion pairs created on the "BCS-side" of a Feshbach resonance. The background shows a classical vortex (Hurricane Isabel in summer 2003, NASA image ISS007E14887).
Vortices in a strongly interacting gas of fermionic atoms on the BEC- and the BCS-side of the Feshbach resonance
At the given field, the cloud of lithium atoms was stirred for 500 ms followed by an equilibration time of 500 ms. After 2ms of ballistic expansion, the magnetic field was ramped below the Feshbach resonance to 735G for imaging. The magnetic fields were 740G (a), 766G (b), 792G (c), 812G (d), 833G (e), 843G (f), 853G (g) and 863G (h). The field of view of each image is 880μm · 880μm.
|The Nature Paper||
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