  We report on the generation of a quantum degenerate Fermi-Fermi mixture of
two different atomic species. The quantum degenerate mixture is realized
employing sympathetic cooling of fermionic Li-6 and K-40 gases by an
evaporatively cooled bosonic Rb-87 gas. We describe the combination of trapping
and cooling methods that proved crucial to successfully cool the mixture. In
particular, we study the last part of the cooling process and show that the
efficiency of sympathetic cooling of the Li-6 gas by Rb-87 is increased by the
presence of K-40 through catalytic cooling. Due to the differing physical
properties of the two components, the quantum degenerate Li-6 K-40 Fermi-Fermi
mixture is an excellent candidate for a stable, heteronuclear system allowing
to study several so far unexplored types of quantum matter.
