# Mixtures of ultracold gases: Fermi sea and Bose-Einstein condensate of Lithium isotopes

Abstract : This thesis presents studies of quantum degenerate
atomic gases of fermionic $^6$Li and bosonic $^7$Li. Degeneracy is
reached by evaporative cooling of $^7$Li in a strongly confining
magnetic trap. Since at low temperatures direct evaporative
cooling is not possible for a polarized fermionic gas, $^6$Li is
sympathetically cooled by thermal contact with $^7$Li. In a first
series of experiments both isotopes are trapped in their low-field
seeking higher hyperfine states. A Fermi degeneracy of
$T/T_F=0.25(5)$ is achieved for $10^5$ fermions. For more than
$\sim 300$ atoms, the $^7$Li condensate collapses, due to the
attractive interatomic interaction in this state. This limits the
degeneracy reached for both species. To overcome this limit, in a
second series of experiments $^7$Li and $^6$Li atoms are
transferred to their low field seeking lower hyperfine states,
where the boson-boson interaction is repulsive but weak. The
inter-isotope collisions are used to thermalize the mixture. A
$^7$Li Bose-Einstein condensate (BEC) of $10^4$ atoms immersed in
a Fermi sea is produced. The BEC is quasi-one-dimensional and the
thermal fraction can be negligible. The measured degeneracies are
$T/T_C=T/T_F=0.2(1)$. The temperature is measured using the
bosonic thermal fraction, which vanishes at the lowest
temperatures, limiting our measurement sensitivity. In a third
series of experiments, the bosons are transferred into an optical
trap and their internal state is changed to $|F=1,m_F=1\rangle$,
the lowest energy state. A Feshbach resonance is detected and used
to produce a BEC with tunable atomic interactions. When the
effective interaction between atoms is tuned to be small and
attractive, we observe the formation of a matter-wave bright
soliton. Propagation of the soliton without spreading over a
macroscopic distance of $1.1\,$mm is observed.
Mots-clés :
Document type :
Theses

Cited literature [195 references]

https://tel.archives-ouvertes.fr/tel-00001340
Contributor : Florian Schreck <>
Submitted on : Friday, May 17, 2002 - 5:31:47 AM
Last modification on : Thursday, October 29, 2020 - 3:01:23 PM
Long-term archiving on: : Tuesday, June 2, 2015 - 3:25:15 PM

### Identifiers

• HAL Id : tel-00001340, version 2

### Citation

Florian Schreck. Mixtures of ultracold gases: Fermi sea and Bose-Einstein condensate of Lithium isotopes. Atomic Physics [physics.atom-ph]. Université Pierre et Marie Curie - Paris VI, 2002. English. ⟨tel-00001340v2⟩

Record views