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

occupation probability
1.0
0.8
0.6
0.4
0.2
2.4. BCS THEORY
0.0
0.0 0.5 1.0
k=
k
1.5 2.0
Figure 2.8: The momentum distribution in a homogeneous gas in the
different limits of BCS theory. On the top the BCS limit, kFa = −1. The
dashed line shows a Fermi gas at the critical temperature for comparison.
The center line is the unitarity limit (1/kFa = 0) and the bottom line
is the BEC limit (kFa = 1). The dotted lines are the predictions from
Astrakharchik et al. [70].
mean field fluctuations into account, revealed that the maximum visible
in the figure should not exist, but the condensation temperature should
increase steadily [69].
2.4.6 Beyond BCS theory
The use of BCS theory throughout the whole BEC-BCS crossover by
Leggett was a bit adventurous, since the BCS approximations only hold
for small attractive interactions. Nevertheless, his approach was justified
later as his theory is a self-consistent theory which predicts experiments
at least qualitatively.
This success had raised the bar for further theoretical research. The
problem is indeed challenging: with the interactions being strong, we
37
F