Ultracold quantum gases are ideal to explore few-body physics because of the tunable two-body interaction. The focus of our research is the exploration of quantum mechanical three-body and four-body systems. For this we investigate inelastic collisions between ultracold Cs atoms and Cs2 molecules. The preparation of the cold atomic sample takes place in an optical dipole trap. We associate the molecules from an ultracold atomic sample by magnetically tunable Feshbach resonances. In this work I report on the implementation of a new dipole trap. The new trap allows us to tune the position and the shape of the dipole potential during the running experiment and to prepare atomic and molecular samples at low temperatures in a convenient way. This new trap opens up a way to many exciting novel experiments. In this thesis I describe the experimental setup focusing on the new system. I also discuss the creation and preparation of Cs2 dimers by Feshbach association from an ultracold atomic sample. The obtained ultracold atomic and molecular sample serves as a starting point to study inelastic atom-dimer and dimer-dimer collisions. Here I will discuss a measurement in which the atom-dimer collision loss coefficient is determined for different external magnetic fields. Future experimental directions will be the research of three-body and four-body physics at high magnetic fields, in particular the investigation of Efimov physics in the ultracold regime.
We demonstrate a generally applicable technique for mixing two-species quantum degenerate bosonic samples in the presence of an optical lattice, and we employ it to produce low-entropy samples of ultracold ^{87}Rb^{133}Cs Feshbach molecules with a lattice filling fraction exceeding 30%. Starting from two spatially separated Bose-Einstein condensates of Rb and Cs atoms, Rb-Cs atom pairs are efficiently produced by using the superfluid-to-Mott insulator quantum phase transition twice, first for the Cs sample, then for the Rb sample, after nulling the Rb-Cs interaction at a Feshbach resonance's zero crossing. We form molecules out of atom pairs and characterize the mixing process in terms of sample overlap and mixing speed. The dense and ultracold sample of more than 5000 RbCs molecules is an ideal starting point for experiments in the context of quantum many-body physics with long-range dipolar interactions.
This thesis investigates universal and non-universal aspects of few-body physics in a mass-imbalanced mixture of ultracold bosonic
133Cs and fermionic
6Li atoms. For this purpose an experimental apparatus for the creation of quantum degenerate gases has been constructed. Using radio-frequency association of weakly-bound LiCs molecules and additional atom-loss spectroscopy we determine the two-body interaction properties between Li and Cs atoms at low collision energies. The analysis with a coupled-channels calculation yields precise singlet and triplet electronic ground state molecular potential curves, from which scattering lengths and positions of Fano-Feshbach resonances are extracted. These results represent almost a tenfold improvement over the previous determination, and are used to study few-body recombination processes and, for the first time, their dependence on the interaction strength between the heavy atoms close to two broad scattering resonances at various temperatures. Measurements of three-body loss rates reveal two series of three and two consecutive LiCs
2 Efimov resonances that demonstrate the universal scaling factor 4.9 between the first and second excited state resonances. The ground state resonance deviates from the discrete scaling law. By the use of hyperspherical formalism with either pairwise zero-range or Lennard-Jones model potentials we identify the van der Waals interaction between Cs atoms as a significant source of these modifications.