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From two-body to many-body physics in an ultracold Bose-Fermi mixture of Li and Cs atoms

Häfner, Stephan Andreas

Published Jan 1, 2018• 1 citation
Publisher

Abstract

This thesis reports on scattering experiments performed with an ultracold mixture of fermionic 6Li and bosonic 133Cs atoms, studying two-, few-, and many-body effects in a highly mass-imbalanced system. To this end, the existing apparatus for the creation of ultracold gases is extended and improved by gray molasses cooling of 6Li and high-resolution absorption imaging. s-, p-, and d-wave Feshbach resonances in the two energetically lowest hyperfine channels are detected via atom-loss and binding energy spectroscopy. These measurements in combination with the coupled channels modeling provide a ten-fold improved parametrization of the interspecies s-wave scattering length and precise determination of the LiCs molecular potential curves. For the first time effects of spin-rotation coupling in Feshbach resonances are observed, reflected in a triplet splitting of the p-wave components. The influence of the intraspecies scattering length on the heteronuclear Efimov scenario is studied by means of threebody loss rate measurements in the vicinity of two Feshbach resonances, characterized either by a positive or negative intraspecies scattering length. The measurements are quantitatively compared to the hyperspherical adiabatic formalism. The positive intraspecies scattering length can be interpreted as three-body parameter. The impact of few-body physics in the many-body impurity problem is discussed and prospects for realization of quantum degenerate Bose-Fermi mixtures with large mass imbalance are provided.

DOI
10.11588/heidok.00023852

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This thesis reports on scattering experiments performed with an ultracold mixture of fermionic 6Li and bosonic 133Cs atoms, studying two-, few-, and many-body effects in a highly mass-imbalanced system. To this end, the existing apparatus for the creation of ultracold gases is extended and improved by gray molasses cooling of 6Li and high-resolution absorption imaging. s-, p-, and d-wave Feshbach resonances in the two energetically lowest hyperfine channels are detected via atom-loss and binding energy spectroscopy. These measurements in combination with the coupled channels modeling provide a ten-fold improved parametrization of the interspecies s-wave scattering length and precise determination of the LiCs molecular potential curves. For the first time effects of spin-rotation coupling in Feshbach resonances are observed, reflected in a triplet splitting of the p-wave components. The influence of the intraspecies scattering length on the heteronuclear Efimov scenario is studied by means of threebody loss rate measurements in the vicinity of two Feshbach resonances, characterized either by a positive or negative intraspecies scattering length. The measurements are quantitatively compared to the hyperspherical adiabatic formalism. The positive intraspecies scattering length can be interpreted as three-body parameter. The impact of few-body physics in the many-body impurity problem is discussed and prospects for realization of quantum degenerate Bose-Fermi mixtures with large mass imbalance are provided.
This thesis reports on the investigation of few-body physics in an \nultracold mass-imbalanced mixture at large scattering lengths. For this purpose, a \nnew apparatus for the creation of quantum degenerate 6Li and 133Cs gases has been \nbuilt. We measure 19 interspecies Feshbach resonances in the two energetically lowest \nhyperfine channels of each species of an ultracold mixture with temperatures on \nthe uK scale. We apply and compare a coupled-channels calculation, the asymptotic \nbound-state model, and the multichannel quantum defect theory for the calculation \nof the Feshbach resonance positions. All models provide a consistent and accurate \ndescription of the two-body scattering resonances in the mixture. Three-body loss \nrate measurements on the negative scattering length side of a broad Feshbach resonance \nin the energetically lowest hyperfine channel at 450 nK reveal two consecutive \nEfimov resonances. The scattering lengths assigned to the two Efimov resonances \nyield a scaling ratio that is consistent with a universal prediction in the zero temperature \nand resonant limit. The presented measurements provide the first test for \nuniversal few-body theories of mixed-species systems with large mass-imbalance.
This thesis investigates universal and non-universal aspects of few-body physics in a mass-imbalanced mixture of ultracold bosonic 133^{133}133Cs and fermionic 6^66Li 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 LiCs2_22​ 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.
This thesis reports on the tunability of interactions in ultracold Bose-Fermi mixtures of Cesium and Lithium. The \nfirst realization of an optically trapped \n6Li -133Cs mixture enabled to perform trap loss spectroscopy measurements to identify \nmagnetic Feshbach resonances. A total of 19 interspecies Feshbach resonances, all in the magnetic field range between 650 G and 950 G, were observed for the two energetically lowest spin states of each species. Two 5 G broad and especially two 60 G broad s-wave resonances give perspectives to produce a dipolar quantum gas of LiCs ground state molecules as well as to study the Efimov effect in highly mass imbalanced systems. In addition, a unique relative tunability of intra- and interspecies \nscattering lengths was found which makes the 6Li -133Cs system also well suited for the investigation of polarons. Evaporative cooling was performed on optically trapped samples which contained only one of the species. In this way, Bose-Einstein condensates of 6Li molecules as well as 133Cs samples at a phase-space density of p = 4 · 10^2 were prepared. All experiments were performed in a new apparatus, which has been designed and set up during this thesis.
This thesis reports measurements of interactions in Fermi-Bose 6Li-133Cs mixtures. Precise control of this Bose-Fermi mixture allowed us to probe few-body physics in regimes which were previously inaccessible. In particular, we performed the first model-independent test of geometric scaling of Efimov physics and probed Efimov resonances farther in the weakly coupled, narrow resonance regime than previously possible.,For this work, we built a new apparatus which overcomes the many challenges faced by Li-Cs mixtures. We developed several novel dipole trapping schemes which overcome the difficulties of mixing Li and Cs, including the large differences in initial trapping and cooling between these atomic species and a large differential gravitational sag. We also achieved part per million level magnetic field control near 900 G, necessary for the precise measurements near narrow Feshbach resonances undertaken in this thesis, by pioneering a tomographic magnetic field calibration technique.,With this apparatus, we first probed the Feshbach resonances of the Li-Cs mixture. This is an essential first step, allowing us to understand and control the two-body interactions between our atoms. Next we began to probe Efimov physics, an important three-body phenomenon wherein an infinite series of three-body bound states arise near two-body scattering resonances, such as Feshbach resonances. We demonstrated the universal scaling expected theoretically for Efimov states near a Feshbach resonance. This task was made feasible in our system by a reduced Efimov scaling constant, yet still required precise magnetic field control. Finally, additional universal behavior of the first Efimov resonance has been observed empirically in a variety of atomic systems. While theory has explained this observed universality, predictions also indicate departures for narrow Feshbach resonances, contrary to previous experimental results. By further improving our magnetic field control to probe a very narrow Feshbach resonance, we have observed a departure from the universal first Efimov resonance, helping to resolve this conflict between experiment and theory.
We report the experimental and theoretical study of two-body interactions in a 6{}^{6}6Li-133{}^{133}133Cs Fermi-Bose mixture. Using a translatable dipole trap setup, we have successfully trapped the two species in the same trap with temperatures of a few microkelvins. By monitoring atom number loss and interspecies thermalization, we identify five sss-wave interspecies Feshbach resonances in the lowest two scattering channels. We construct a coupled channels model using molecular potentials to fit and characterize these resonances. Two of the resonances are as wide as 6 mT (60 G) and thus should be suitable for creating Feshbach molecules and searching for universal few-body scaling.
We report on the observation of 19 interspecies Feshbach resonances in an optically trapped ultracold Bose-Fermi mixture of 133Cs{}^{133}\text{Cs}133Cs and 6Li{}^{6}\text{Li}6Li in the two energetically lowest spin states. We assign the resonances to sss- and ppp-wave molecular channels by a coupled-channels calculation, resulting in an accurate determination of LiCs ground-state potentials. Fits of the resonance position based on the undressed asymptotic bound state model do not provide the same level of accuracy as the coupled-channels calculation. Several broad sss-wave resonances provide prospects to create fermionic LiCs molecules with a large dipole moment via Feshbach association followed by stimulated Raman passage. Two of the sss-wave resonances overlap with a zero crossing of the Cs scattering length, which offers prospects for the investigation of polarons in an ultracold Li-Cs mixture.