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

This thesis presents studies of quantum degenerate atomic gases of fermionic Li-6 and bosonic Li-7. Degeneracy is reached by evaporative cooling of Li-7 in a strongly confining magnetic trap. Since at low temperatures direct evaporative cooling is not possible for a polarized fermionic gas, Li-6 is sympathetically cooled by thermal contact with Li-7. 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 similar to300 atoms, the Li-7 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 Li-7 and Li-6 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 Li-7 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], 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.