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Quick Links |
May 2007:
We used radio-frequency spectroscopy to study pairing in the normal and superfluid phases of a strongly interacting Fermi gas with imbalanced spin populations. At high spin imbalances, the system does not become superfluid even at zero temperature. In this normal phase, full pairing of the minority atoms was observed. Hence, mismatched Fermi surfaces do not prevent pairing but can quench the superfluid state, thus realizing a system of fermion pairs that do not condense even at the lowest temperature.
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The temperature-imbalance diagram
shows where the rf spectra presented in this work were taken. All spectra
were obtained on resonance at 833 G. The
arrows indicate the order in which the spectra are
displayed in the figures. The shaded region indicates
the superfluid phase. Except for the data close to zero imbalance, for which the interacting temperature T´ is given, temperatures have been determined from the noninteracting wings of the majority cloud. |
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Radio-frequency spectra of the minority component obtained while
crossing the phase transition by reducing imbalance (A to C) and temperature
(D to F). The rf spectra do not reveal the phase transition. The onset of superfluidity is indirectly observed by fermion pair condensation. The onset of superfluidity as a function of temperature occurs between (D) and (F). The insets in (A) to (F) show the column density profile (red) of the minority cloud after a rapid magnetic field ramp to the BEC side and further expansion. The additional insets in (D) to (F) show phase-contrast images for a trapped cloud, obtained at imbalances of the opposite sign. |
February 2007:
We have studied the expansion of a rotating, superfluid Fermi gas. The presence and absence of vortices in the rotating gas were used to distinguish the superfluid and normal parts of the expanding cloud. We found that the superfluid pairs survive during the expansion until the density decreases below a critical value. Our observation of superfluid flow in the expanding gas at 1/kFa = 0 extends the range where fermionic superfluidity has been studied to densities of 1.2 x 1011 cm-3, about an order of magnitude lower than any previous study.
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Shown are absorption images for different expansion times on the BCS side of the Feshbach resonance at 910 G (0.0, 1.0, 2.0, 3.0, 3.5, 4.0, and 4.5 ms) and 960 G (0.0, 0.5, 1.0, 1.5, 2.0, 2.5, and
3 ms), before the magnetic field was ramped to the BEC side for further expansion. The vortices served as markers for the superfluid parts of the cloud. Superfluidity survived the expansion for several milliseconds and was gradually lost from the low density edges of the cloud towards its center. Compared to 910 G (a=7200 a0), superfluidity decayed faster at 960 G (a=5000 a0) due to the reduced interaction strength. |
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Starting at a peak kFa in the optical trap
(triangles) vortices survived up to a critical peak kFa of -0.8 ± 0.1 (squares), almost independent of the magnetic field (scattering
length). Solid circles correspond to partially superfluid, open circles to normal clouds. The observed number of vortices is color coded. The critical kFa was obtained for each magnetic field separately by taking the average of the peak kF of the last partially superfluid and the first completely normal cloud. |
July 2006:
We have observed phase separation between the superfluid and the normal component in a strongly interacting Fermi gas with imbalanced spin populations. The in situ distribution of the density difference between two trapped spin components is obtained using phase-contrast imaging and 3D image reconstruction. A shell structure is clearly identified where the supefluid region of equal densities is surrounded by a normal gas of unequal densities. The phase transition induces a dramatic change in the density profiles as excess fermions are expelled from the superfluid.
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(a) The probe beam
is tuned to the red for the |1> to |e> transition and to te blue for the |2> to |e>
transition. the resulting optical signal in the phase contrast image is proportional
to the density difference of the atoms in the two atomic states. (b) Phase contrast images of trapped atomic clouds in state |1> (left) and state |2> (right) and of and equal mixture of the two states (center). |
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As a sample with imbalanced population in the two spin states is cooled down, the
formation of a region with equal densities in the two spin states can be observed. The temperature of the cloud was controlled by varying the final value of the trap depth Uf in the evaporation process. The whole evaporation process was performed in the unitary regime at a magnetic field of 834G. |
June 2006:
Water freezes into ice, atomic spins spontaneously align in a magnet, liquid helium becomes superfluid: Phase transitions are dramatic phenomena. However, despite the drastic change in the system's behaviour, observing the transition can sometimes be subtle. The hallmark of Bose-Einstein condensation (BEC) and superfluidity in trapped, weakly interacting Bose gases is the sudden appearance of a dense central core inside a thermal cloud. In strongly interacting gases, such as the recently observed fermionic superfluids, this clear separation between the superfluid and the normal parts of the cloud is no longer given. Condensates of fermion pairs could be detected only using magnetic field sweeps into the weakly interacting regime. The quantitative description of these sweeps presents a major theoretical challenge. Here we demonstrate that the superfluid phase transition can be directly observed by sudden changes in the shape of the clouds, in complete analogy to the case of weakly interacting Bose gases. By preparing unequal mixtures of the two spin components involved in the pairing, we greatly enhance the contrast between the superfluid core and the normal component. Furthermore, the non-interacting wings of excess atoms serve as a direct and reliable thermometer. Even in the normal state, strong interactions significantly deform the density profile of the majority spin component. We show that it is these interactions which drive the normal-to-superfluid transition at the critical population imbalance of 70(5)%
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Top a-c and bottom d-f rows show the normal and the superfluid state, respectively. The appearance of a dense central feature in the smaller component marks the onset of condensation. The condensate causes a clear depletion in the difference profiles (bottom of each panel). Both in the normal and in the superfluid state, interactions between the two spin states are manifest in the strong deformation of the larger component. The dashed lines show Thomas-Fermi fits to the wings of the column density. |
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The curvature of the observed column density is encoded in shades of gray with white (black) corresponding to positive (negative) curvature. The outer radii of the two components and the condensate radius are shown as an overlay in the lower panel. As a direct consequence of strong interactions, the minority component causes a pronounced bulge in the majority density that is reflected in the rapid variation of the profile's curvature. The condensate is clearly visible in the minority component (± > 0), but also leaves a faint trace in the minority component (± < 0). |