Martin Teichmann Atomes de lithium-6 ultra froids dans la ... - TEL

ellipticity
1.4
1.3
1.2
1.1
1.0
0.9
5.3. HYDRODYNAMIC EXPANSION
0.0 0.1 0.2 0.3
t = ms
0.4 0.5 0.6
Figure 5.13: The ellipticity of the cloud for various times in the magnetic
field at the unitarity limit. The expected ellipticity for long expansion times
is 1,4. This is the same dataset as figure 5.6, the error bars are calculated
in the same way.
to be imaged. In order to verify the validity of our data, we repeated the
experiment, stopping the magnetic field after different times, while leaving
the expansion without field constant at 0,5 ms. The result is shown in
figure 5.13. In this figure, we see that the cloud already reaches its final
ellipticity after 0,2 ms. It is worth noting that this measurement does not
correspond to figure 5.10. The two curves present different situations:
the experimental data have an additional expansion time without field
during which the ellipticity changes. The point at zero expansion time
in field is actually a momentum distribution measurement. This is not
true for the other points, as one cannot expect the cloud which already
expanded in field to be small compared to the final size. This is necessary
for the momentum distribution measurements to be valid.
We repeated the same curve using the evaporation scheme described
for the momentum distribution, shown in figure 5.14. This way we have
more atoms left, leading to a better signal to noise ratio. The temperature
of the gas is significantly higher. A Fermi fit to the noninteracting gas
gave T = 0,3TF. At this temperature we can only expect to be condensed
very close to the unitarity limit, as we see from figure 2.9. And indeed,
we observe that the step seen at lower temperatures has disappeard.
We are near the predicted ellipticity for a hydrodynamic gas at the
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