Towards strongly interacting Fermi-Fermi mixtures of ultracold atoms*

Strongly interacting mixtures of unequal fermionic species promise to allow access to novel states of matter, such as Cooper pairing without time-reversal symmetry, the FFLO (Fulde-Ferrell-Larkin- Ovchinnikov) state of Cooper pairs at finite momenta, and a basic form of quark (color) superfluidity. We will present our experiments on cooling a Fermi-Fermi mixture of the fermionic Alkalis $^6$Li and $^{40}$K. The difficulty of the low natural abundance of $^{40}$K (\%0.01) is typically overcome with the use of enriched, but expensive, $^{40}$K. In our approach we use two independent Zeeman slowers optimized for high atomic fluxes of non-enriched K and Li. This allows us to load 5 x $10^7$ fermionic K into a magneto-optical trap, and it also gives us access to the bosonic isotopes of $^{39}$K and $^{41}$K as possible sympathetic coolants for both $^6$Li and $^{40}$K. As a crucial step, we have produced a Bose-Einstein condensate of $^{41}$K by direct evaporation and we were able to sympathetically cool the fermion $^{40}$K. Our immediate goal is the production of a degenerate Fermi-Fermi mixture of $^6$Li and $^{40}$K.