Ultracold Fermi-Fermi Mixtures of Lithium and Potassium

Ultracold atomic Fermi gases are a unique experimental tool for simulating and studying many-body systems. Since they are very well controllable and clean systems with tunable interactions, they can serve as quantum simulators for effects that occur in solid states and usually arise from the quantum nature of the Fermi gas of electrons. Different phenomena such as high-temperature superconductivity, Josephson junctions and ferromagnetism can be explored using degenerate Fermi gases. Even in the context of particle physics, Fermi gases can be used to simulate the behavior of quarks inside a nucleus. In the past decade, experiments with a single fermionic (either 6Li or 40K) species have led to ground breaking results. In the past five years, several experiments have been set up that combine the two fermionic alkali species. Additionally to the capabilities offered by a single-species experiment (such as tuning of interaction strength, spin polarization, trap parameters), the two-species mixtures open up control of new parameters - the most obvious being the mass ratio. Due to the mass-imbalance, the Fermi-spheres would no longer overlap, and thus exotic quantum phases emerge. So far experimentally unobserved effects include superfluidity, phase separation, crystalline phases, exotic pairing mechanisms and long-lived trimers. More practically, a mixture would allow to species-selectively apply optical trapping potentials. This thesis presents the experimental efforts from creating an ultracold Fermi- Fermi mixture of 6Li and 40K to the creation of heteronuclear molecules. Three published articles are contained in this thesis. In the first article we report on the observation of Feshbach resonances in an ultracold mixture of two fermionic species, 6Li and 40K. The experimental data are interpreted using a simple asymptotic bound state model and full coupled channels calculations. This unambiguously assigns the observed resonances in terms of various s- and p-wave molecular states and fully characterizes the ground-state scattering properties in any combination of spin states. In the second article we investigate the collisional stability of a sample of 40K atoms immersed in a tunable spin mixture of 6Li atoms. In this three-component Fermi-Fermi mixture, we find very low loss rates in a wide range of interactions as long as molecule formation of 6Li is avoided. The stable fermionic mixture with two resonantly interacting spin states of one species together with another species is a promising system for a broad variety of phenomena in few- and many-body quantum physics. In the third article we present the essential experimental steps of our all-optical approach to prepare a double-degenerate Fermi-Fermi mixture of 6Li and 40K atoms, which then serves as a starting point for molecule formation. We first describe the optimized trap loading procedures, the internal-state preparation of the sample, and the combined evaporative and sympathetic cooling process. We then discuss the preparation of the sample near an interspecies Feshbach resonance, and we demonstrate the formation of heteronuclear molecules by a magnetic field ramp across the resonance. (author)