This thesis reports on experiments studying the few-body physics of three distinguishable fermionic atoms with large scattering lengths. The experiments were performed with ultracold gases of fermionic 6Li atoms in three different hyperfine states. By tuning the strength of the interactions between the atoms with Feshbach resonances and measuring the rate constants for inelastic three-atom and atom-dimer collisions the intersections of two universal trimer states with the three-atom and atom-dimer continuum could be located. Subsequently, one of these Efimov states was directly observed with RF-association spectroscopy. Using this technique the binding energy of this Efimov state was measured as a function of the strength of the interparticle interactions. The experiments presented in this thesis provide a nearly complete understanding of the universal few-body physics of three-component Fermi gases of 6Li atoms. This understanding will be the foundation for future studies of the many-body physics of three-component Fermi gases.
This thesis reports on experiments studying the few-body physics of three distinguishable fermionic atoms with large scattering lengths. The experiments were performed with ultracold gases of fermionic 6Li atoms in three different hyperfine states. By tuning the strength of the interactions between the atoms with Feshbach resonances and measuring the rate constants for inelastic three-atom and atom-dimer collisions the intersections of two universal trimer states with the three-atom and atom-dimer continuum could be located. Subsequently, one of these Efimov states was directly observed with RF-association spectroscopy. Using this technique the binding energy of this Efimov state was measured as a function of the strength of the interparticle interactions. The experiments presented in this thesis provide a nearly complete understanding of the universal few-body physics of three-component Fermi gases of 6Li atoms. This understanding will be the foundation for future studies of the many-body physics of three-component Fermi gases.
This thesis reports on experimental studies of few-body physics in ultracold Fermi gases. The main part is devoted to the first realisation of an ultracold three-component Fermi gas in thermal equilibrium that consists of atoms in the three lowest spin states of 6Li. In first experiments the collisional stability of the gas was investigated in dependence of the interparticle interaction. The interaction strength was tuned by an external magnetic field employing Feshbach resonances in all three two-body scattering channels. At two magnetic field values a resonant enhancement of three-body loss was observed that can be explained by an Efimov-like trimer consisting of one atom in each of the three states which crosses the continuum twice in the studied magnetic field region. The behaviour of three-body loss rates could be understood by taking into account a variation of the lifetime of the trimer with the magnetic field. These experiments were performed with large ensembles of atoms. A finite deeply degenerate Fermi gas consisting of a controlled number of atoms may serve as a clean model for finite Fermi systems as for example atomic nuclei. An approach for the preparation of such a gas is discussed and first experimental steps are presented.
Ultracold gases of three distinguishable particles with large scattering lengths are expected to show rich few-body physics related to the Efimov effect. We have created three different mixtures of ultracold 6Li atoms and weakly bound 6Li2 dimers consisting of atoms in three different hyperfine states and studied their inelastic decay via atom-dimer collisions. We have found resonant enhancement of the decay due to the crossing of Efimov-like trimer states with the atom-dimer continuum in one mixture as well as minima of the decay in another mixture, which we interpret as a suppression of exchange reactions of the type |12+|3→|23+|1. Such a suppression is caused by interference between different decay paths and demonstrates the possibility of using Efimov physics to control the rate constants for molecular exchange reactions in the ultracold regime.
Ultracold atomic gases with tunable interactions offer an ideal platform for studying interacting quantum matter. While the few- and many-body physics are generally complex and intractable, the problem can be greatly simplified in an atomic gas by a controlled separation of relevant length and energy scales. Precise control of experimental parameters, via Feshbach resonances, optical potentials and radio-frequency radiation, enables deterministic measurements of few-body physics, including universal physics and the Efimov effect. In this thesis, I present our recent studies on precisely measuring two- and three-body physics in an ultracold Bose gas. I begin by describing our new apparatus used for generating and studying ultracold 39K gas samples. Then, I focus on our precision spectroscopy of Feshbach dimer binding energies, spanning three orders of magnitude and with sub-kilohertz resolution. These measurements enable us to locate a Feshbach resonance and determine scattering length values with unprecedented accuracy. Finally, I present our precise measurements locating an exotic three-body state, specifically the Efimov ground state. We find that the trimer state location significantly deviates from the value predicted by van der Waals universality. Due to small experimental and systematic uncertainties, our measurement is the strongest evidence of departure from the universal value and is the first observed deviation near a Feshbach resonance of intermediate strength.
Few-body effects play an important role for the understanding of ultracold quantum gases. We make use of an effective field theory approach to investigate various aspects of universal few-body physics close to a Feshbach resonance. That is the regime where the scattering length is large compared to all other length scales of the system and thus determines the observables. It is also the regime where the Efimov effect whose main characteristic is the occurrence of a sequence of three-body bound states becomes important. At unitarity, i.e., for diverging scattering length, the ratio of binding energies of neighboring states approaches a constant. The existence of those trimers can be captured by a single three-body parameter and influences observables such as recombination rates. Starting from adequate Lagrangians, we derive atom-dimer scattering amplitudes that contain all information of interest about the three-particle systems. With this method, we first investigate the scattering of atoms and dimers at finite temperature in the presence of the Efimov effect. We calculate the dimer relaxation rate and obtain good agreement with available experimental data. Furthermore, heteronuclear mixtures exhibiting large interspecies scattering length are studied in detail. If bosons are the majority species, the Efimov effect occurs in an S-wave channel and we are able to compute three-body recombination and dimer relaxation rates. The results are compared to the outcome of two existing experiments. For mainly fermionic systems, the Efimov effect is only present in an overall P-wave and three-body recombination at threshold vanishes. However, in mixtures of atoms and dimers, scattering cross sections and dimer relaxation show the typical Efimov behavior of log-periodicity. We also consider two-dimensional Bose gases, where the Efimov effect does not occur. For those systems, we derive an equation including the first non-universal corrections and deduce three-body observables such as binding energies and atom-dimer scattering parameters.
This thesis presents experiments exploring Feshbach molecules and Efimov trimers in an ultracold Bose-Fermi atomic gas mixture of 40K and 87Rb. These bound states are accessed via scattering resonances, called Fano-Feshbach resonances, which arise as a consequence of a bound state being tuned through degeneracy with the threshold energy of colliding atoms. In addition to allowing the tunability of interactions, the Fano-Feshbach resonance allows for a weakly bound molecule state, called the Feshbach molecule state, to be populated. The efficiency of conversion to the Feshbach molecule state depends upon the rate with which the bound state energy is swept through degeneracy with atomic threshold energy. In regions of large scattering lengths, three-body bound states, called Efimov states, also influence inelastic Bose-Bose-Fermi collisions and boson+molecule collisions in our system. The work presented in this thesis utilizes a magnetically tunable Fano-Feshbach resonance to study Feshbach molecule creation in the perturbative limit (the limit of fast magnetic-field sweeps) and the saturated limit (the limit of slow sweeps), where a phenomenological model is applied to understand the conversion efficiency as well as the quantum degeneracy of the molecules. In addition, I present a study of boson+molecule collision rates and Bose-Bose-Fermi collision rates in order to observe signatures of Efimov states and compare against results in the gas mixture 41K-87Rb to address the question of universality of the three-body parameter in heteronuclear gas mixtures.
Few-Body Problem Seemingly simple, quantum mechanical few-body systems are notoriously difficult to describe. Efimov trimers, three-body bound states with interactions tuned to be in close vicinity of the formation of two-body bound states, are the most tractable of these systems, with relevance, for example, in nuclear physics. Observed recently in ultracold atomic gases through their signatures in the rate of inelastic three-body collisions, Efimov trimers are predicted to appear at interaction strengths whose ratios are universally specified. By measuring binding energy as a function of interaction strength, Lompe et al. (p. 940 ) directly observed the association of three distinguishable atoms into a bound state. This technique may enable more precise studies of the trimer state, potentially revealing the nature of nonuniversal corrections suggested by prior experiments.