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

This thesis presents studies of quantum degenerate atomic gases of fermionic ⁶Li and bosonic ⁷Li. Degeneracy is reached by evaporative cooling of ⁷Li in a strongly confining magnetic trap. Since at low temperatures direct evaporative cooling is not possible for a polarized fermionic gas, ⁶Li is sympathetically cooled by thermal contact with ⁷Li. 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 10⁵ fermions. For more than ∼ 300 atoms, the ⁷Li 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 ⁷Li and ⁶Li 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 ⁷Li Bose-Einstein condensate (BEC) of 10⁴ 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), 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. (author)