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arXiv:2001.05329

Magnetic Feshbach resonances in $^{7}\text{Li}-^{133}\text{Cs}$ mixtures

Pascal Naidon

Published Jan 15, 2020• 1 citation
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Abstract

Motivated by the prospect of observing Efimov and Bose polaron physics in ultracold mixtures of bosonic atoms with large mass imbalance, this work investigates the magnetic Feshbach resonances between $^{7}\text{Li}$ and $^{133}\text{Cs}$. The resonances are predicted at the 1 gauss level using the model of Pires et al. [Phys. Rev. A, 90, 012710] obtained from experimental observations of resonances between $^{6}\text{Li}$ and $^{133}\text{Cs}$. It is found that a few resonances in a practical range of magnetic field intensity could be used to tune the scattering length between $^{7}\text{Li}$ and $^{133}\text{Cs}$ atoms. Opportunities for observing Efimov and Bose polaron physics are discussed.

DOI
10.48550/arxiv.2001.05329

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Motivated by the prospect of observing Efimov and Bose polaron physics in ultracold mixtures of bosonic atoms with large mass imbalance, this work investigates the magnetic Feshbach resonances between 7Li^{7}\text{Li}7Li and 133Cs^{133}\text{Cs}133Cs. The resonances are predicted at the 1 gauss level using the model of Pires et al. [Phys. Rev. A, 90, 012710] obtained from experimental observations of resonances between 6Li^{6}\text{Li}6Li and 133Cs^{133}\text{Cs}133Cs. It is found that a few resonances in a practical range of magnetic field intensity could be used to tune the scattering length between 7Li^{7}\text{Li}7Li and 133Cs^{133}\text{Cs}133Cs atoms. Opportunities for observing Efimov and Bose polaron physics are discussed.
We report on the experimental observation of Feshbach resonances in large-mass-imbalance mixtures of erbium (Er) and lithium (Li). All combinations of 168Er,166Er^{168}\mathrm{Er}, ^{166}\mathrm{Er}168Er,166Er, and 7Li,6Li^{7}\mathrm{Li}, ^{6}\mathrm{Li}7Li,6Li are cooled to temperatures of a few microkelvin, partially by means of sympathetic cooling together with ytterbium (Yb) as a third mixture component. The Er-Li inelastic interspecies collisional properties are studied for magnetic fields up to 680G680\phantom{\rule{0.28em}{0ex}}\mathrm{G}680G. In all cases, resonant interspecies loss features, indicative of Feshbach resonances, have been observed. While most resonances have sub-Gauss widths, a few of them are broad and feature widths of several Gauss. Those broad resonances are a key to the realization of ultracold Er-Li quantum gas mixtures with tunable interactions.
We study Efimov resonances via three-body loss in an ultracold two-component gas of fermionic 6Li^{6}\mathrm{Li}6Li and bosonic 133Cs^{133}\mathrm{Cs}133Cs atoms close to a Feshbach resonance at 843 G, extending results reported previously [Pires et al., Phys. Rev. Lett. 112, 250404 (2014)] to temperatures around 120 nK. The experimental scheme for reaching lower temperatures is based upon compensating the gravity-induced spatial separation of the mass-imbalanced gases with bichromatic optical dipole traps. We observe the first and second excited Li-Cs-Cs Efimov resonance in the magnetic field dependence of the three-body event rate constant, in good agreement with the universal zero-range theory at finite temperature [Petrov and Werner, Phys. Rev. A 92, 022704 (2015)]. Deviations are found for the Efimov ground state, and the inelasticity parameter $\ensuremath{\eta}$ is found to be significantly larger than those for single-species systems.
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.
We present an analysis of experimentally accessible magnetic Feshbach resonances in ultracold heteronuclear $^{85}\mathrm{Rb}\text{\ensuremath{-}}^{87}\mathrm{Rb}$ and $^{6}\mathrm{Li}\text{\ensuremath{-}}^{87}\mathrm{Rb}$ mixtures. Using recent experimental measurements of the triplet scattering lengths for $^{6}\mathrm{Li}\text{\ensuremath{-}}^{87}\mathrm{Rb}$ and $^{7}\mathrm{Li}\text{\ensuremath{-}}^{87}\mathrm{Rb}$ mixtures and Feshbach resonances for one combination of atomic states, we create model potential curves and fine-tune them to reproduce the measured resonances and to predict the location of several experimentally relevant resonances in Li-Rb collisions. To model $^{85}\mathrm{Rb}\text{\ensuremath{-}}^{87}\mathrm{Rb}$ collisions, we use accurate Rb2{\mathrm{Rb}}_{2}Rb2​ potentials obtained previously from the analysis of experiments on $^{87}\mathrm{Rb}\text{\ensuremath{-}}^{87}\mathrm{Rb}$ collisions. We find resonances that occur at very low magnetic fields, below 10G10\phantom{\rule{0.3em}{0ex}}\mathrm{G}10G, which may be useful for entanglement generation in optical lattices or atom chip magnetic traps.
We observe two consecutive heteronuclear Efimov resonances in an ultracold Li-Cs mixture by measuring three-body loss coefficients as a function of magnetic field near a Feshbach resonance. The first resonance is detected at a scattering length of a_((0))=-320(10)a_((0)), corresponding to ∼7(∼3) times the Li-Cs (Cs-Cs) van der Waals range. The second resonance appears at 5.8(1.0)a_((0)), close to the unitarity-limited regime at the sample temperature of 450 nK. Indication of a third resonance is found in the atom loss spectra. The scaling of the resonance positions is close to the predicted universal scaling value of 4.9 for zero temperature. Deviations from universality might be caused by finite-range and temperature effects, as well as magnetic field-dependent Cs-Cs interactions.
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.