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January 2006:
We established superfluidity in a two-state mixture of ultracold fermionic atoms with imbalanced state populations. This study relates to the long-standing debate about the nature of the superfluid state in Fermi systems. Indicators for superfluidity were condensates of fermion pairs and vortices in rotating clouds. For strong interactions, near a Feshbach resonance, superfluidity was observed for a broad range of population imbalances. We mapped out the superfluid regime as a function of interaction strength and population imbalance and characterized the quantum phase transition to the normal state, known as the Pauli limit of superfluidity.
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Vortices as a tool of Metrology: When is the cloud superfluid? Preparation of several spin-mixtures at 853G and 812G, respectively. Before evaporation and stirring, the population transfer from spin state |1> to spin state |2> is varied from 0% to 100% by the Landau-Zener ramp speed. |
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Radial density profiles of the two components of a strongly interacting Fermi gas mixture with unequal populations. The profiles are azimuthal averages of the axially integrated density. A) and B): Profiles of the component in state |1> and |2>, respectively, originating from 883 G (1/kFa =-0.27). The population imbalance was δ=0% (red), δ=46% (blue), and δ=86% (green). (C) Difference between the distributions in state |1> and |2> . The clear dip in the blue curve caused by the pair condensate indicates phase separation of the superfluid from the normal gas. (D) Color-coded profiles of clouds prepared at three different interaction strengths. The condensate is clearly visible as the dense central part surrounded by unpaired fermions or uncondensed molecules. |
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Transition from the Superfluid to the Normal State. Critical difference in Fermi energies δEF between the two spin states for which the superfluid to normal transition is observed. δEF for each interaction strength and temperature is obtained from the critical population imbalance determined in the figure above. Representative density profiles illustrate the quantum phase transition for fixed interaction and for fixed population imbalance along the dashed lines. |
May 2005:
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.
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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). |
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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. |
December 2004:
The dynamics of pair condensate formation in a strongly interacting Fermi gas close to a Feshbach resonance was studied. We employed a phase-shift method in which the delayed response of the manybody system to a modulation of the interaction strength was recorded. The observable was the fraction of condensed molecules in the cloud after a rapid magnetic field ramp across the Feshbach resonance. The measured response time was slow compared to the rapid ramp, which provides final proof that the molecular condensates reflect the presence of fermion pair condensates before the ramp.
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Imaging of molecular condensates. The rapid ramp to zero field after release from the trap created a cloud containing both molecules and unpaired atoms. A Stern-Gerlach field gradient separated atoms (magnetic moment ±1/3µB for states |1> and |2>, respectively) from molecules, which are purely singlet at zero field. At the end of 5 ms of ballistic expansion, the molecules were dissociated in a fast ramp (in 3 ms to 1200 G) across the Feshbach resonance. After another 2 ms expansion again at zero field, an absorption image of the separated clouds was taken. Condensate fractions were determined from the molecular cloud, and the numbers in each component were recorded. An absorption image is shown on the bottom, the field of view is 3 mm x 1 mm. |
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Shown is the delayed response of the observed condensate fraction (data points and thick line to guide the eye) to a 250 Hz magnetic field modulation (thin line) on the BCS side of the Feshbach resonance at 834 G. The vertical lines indicate the points of maximum condensate fraction, which are delayed with respect to the times at which the magnetic field is closest to resonance. For our trap parameters, the response was delayed by 500µs. This time is far longer than the time spent within the resonance region during the conversion of fermion pairs into molecules. This provides final proof that the observed molecular condensates originated from condensates of pairs of fermions above the resonance. |
| Condensation of Fermion Pairs Near a Feshbach Resonance | Observation of Bose-Einstein Condensation of Molecules |
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| Feshbach Resonances in Fermionic 6Li | Observation of Feshbach Resonances between Two Different Atomic Species |
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| Spectroscopic Insensitivity to Cold Collisions in a Two-State Mixture of Fermions | Fiftyfold Improvement in the Number of Quantum Degenerate Fermionic Atoms |
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| Radio-Frequency Spectroscopy of Ultracold Fermions | Two-Species Mixture of Quantum Degenerate Bose and Fermi Gases |
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| Decay of Ultracold Fermionic Lithium Gas Near a Feshbach Resonance | Collisions in zero temperature Fermi gases |
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