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

Chapter 5
Experimental results
Well roared, Lion.
– W. Shakespeare A Midsummer Night’s Dream
The heart of an experimental physics thesis is the presentation
of the results achieved in the experiment. Our interest is to study
superfluidity in the BEC-BCS crossover. The two main sections of this
chapter are devoted to the study of the momentum distribution and the
hydrodynamic expansion of a gas in the crossover region. An outline of
similar experiments reported by other groups will be given. In addition,
we developed a new analysis of an older experiment on molecular BECs.
We accompany these experiments with measurements of the creation
of molecules happening while crossing the Feshbach resonance. We
extended the older experiments using slow magnetic field sweeps to
high sweep rates.
Finally results on heteronuclear Feshbach resonances are presented,
with initial measurements on this interesting extension of our current
work.
5.1 Momentum distribution
One of the main ingredients to BCS theory is the prediction of the the
momentum distribution for superfluid fermions: even at zero temperature,
it does not follow a Fermi-Dirac distribution as one would expect
but the step of this distribution is smeared out. At zero temperature, the
momentum distribution resembles closely the Fermi distribution of a
non-interacting gas at the condensation temperature. The momentum
distribution is accessible in experiments (this is illustrated in figure 2.8).
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