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Chinese Physics Letters

Measurement of S-Wave Scattering Length between 6Li and 88Sr Atoms Using Interspecies Thermalization in an Optical Dipole Trap*

Xiaobin Ma, Zhu-Xiong Ye, Li-Yang Xie, Zhen Guo, Li You, Meng Khoon Tey

Published Jul 1, 2019• 4 citations
Publisher

Abstract

We report the creation of the first mixture of 6Li and 88Sr atoms in an optical dipole trap. Using this mixture, a measurement of the interspecies thermalization process is carried out and the previously unknown interspecies s-wave scattering length between 6Li and 88Sr atoms is extracted to be | a 6 L i − 88 S r | = ( 380 − 100 + 160 ) a 0 with a0 being the Bohr radius from the rate of interspecies thermalization.

DOI
10.1088/0256-307x/36/7/073401

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We report the creation of the first mixture of 6Li and 88Sr atoms in an optical dipole trap. Using this mixture, a measurement of the interspecies thermalization process is carried out and the previously unknown interspecies s-wave scattering length between 6Li and 88Sr atoms is extracted to be | a 6 L i − 88 S r | = ( 380 − 100 + 160 ) a 0 with a0 being the Bohr radius from the rate of interspecies thermalization.
We present measurements of interspecies thermalization between ultracold samples of 133Cs^{133}\mathrm{Cs}133Cs and either 174Yb^{174}\mathrm{Yb}174Yb or 170Yb^{170}\mathrm{Yb}170Yb. The two species are trapped in a far-off-resonance optical dipole trap and 133Cs^{133}\mathrm{Cs}133Cs is sympathetically cooled by Yb. We extract effective interspecies thermalization cross sections by fitting the thermalization measurements to a kinetic model, giving ${\ensuremath{\sigma}}_{\mathrm{Cs}^{174}\mathrm{Yb}}=\left(5\ifmmode\pm\else\textpm\fi{}2\right)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{2}$ and ${\ensuremath{\sigma}}_{\mathrm{Cs}^{170}\mathrm{Yb}}=\left(18\ifmmode\pm\else\textpm\fi{}8\right)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{2}$. We perform quantum scattering calculations of the thermalization cross sections and optimize the CsYb interaction potential to reproduce the measurements. We predict scattering lengths for all isotopic combinations of Cs and Yb. We also demonstrate the independent production of 174Yb^{174}\mathrm{Yb}174Yb and 133Cs^{133}\mathrm{Cs}133Cs Bose-Einstein condensates using the same optical dipole trap, an important step toward the realization of a quantum-degenerate mixture of the two species.
We report on the realization of a stable mixture of ultracold lithium and ytterbium atoms confined in a far-off-resonance optical dipole trap. We observe sympathetic cooling of 6Li^{6}\mathrm{Li}6Li by 174Yb^{174}\mathrm{Yb}174Yb and extract the sss-wave scattering length magnitude $|{a}_{^{6}\mathrm{Li}\mathrm{\text{\ensuremath{-}}}^{174}\mathrm{Yb}}|=(13\ifmmode\pm\else\textpm\fi{}3){a}_{0}$ from the rate of interspecies thermalization. Using forced evaporative cooling of 174Yb^{174}\mathrm{Yb}174Yb, we achieve reduction of the 6Li^{6}\mathrm{Li}6Li temperature to below the Fermi temperature, purely through interspecies sympathetic cooling.
Thermalization measurements on an ultracold mixture of metastable 4^44He and 87^{87}87Rb atoms in a quadrupole magnetic trap
Recently we have reported (Knoop et al. [arXiv:1404.4826]) on an experimental determination of metastable triplet 4^44He+87^{87}87Rb scattering length by performing thermalization measurements for an ultracold mixture in a quadrupole magnetic trap. Here we present our experimental apparatus and elaborate on these thermalization measurements. In particular we give a theoretical description of interspecies thermalization rate for a quadrupole magnetic trap, i. e. in the presence of Majorana heating, and a general procedure to extract the scattering length from the elastic cross section at finite temperature based on knowledge of the C6C_6C6​ coefficient alone. In addition, from our thermalization data we obtain an upper limit of the total interspecies two-body loss rate coefficient of 1.5×10−121.5\times 10^{-12}1.5×10−12 cm3^33s−1^{-1}−1.
This thesis describes trapping and evaporative cooling of ultracold 87Sr and 88Sr mixtures in an optical dipole trap to produce the first Bose-Einstein condensate (BEC) of 88Sr. Furthermore, this work presents thermalization studies that characterize the scattering properties of these ultracold strontium samples. Such ultracold atomic gases have become an important area of research because of their potential for improving optical frequency standards and for realizing quantum computation using neutral atoms. A BEC of 88Sr is particularly interesting because the small value of its background s-wave scattering length may enable the use of optical Feshbach resonances to create two-dimensional solitons. However, the small scattering length for 88Sr also hinders efficient evaporative cooling in the optical dipole trap, a necessary step to producing a BEC. Experiments with other ultracold gases have successfully overcome this hurdle by mixing in a second atomic species which, by introducing stronger interactions with the weakly interacting species, enables evaporation to colder temperatures via sympathetic cooling. For this work, we use 87Sr to sympathetically cool 88Sr during forced evaporation to quantum degeneracy.\nPrevious experiments in the Killian Lab characterized 88Sr in detail. Here, I emphasize the new or improved aspects that have allowed trapping and cooling of the mixtures of 87Sr and 88 Sr: trapping of 87Sr by itself, spectroscopic measurements of all the stable strontium isotopes to guide the trapping of isotopic mixtures, imaging of both 87Sr and 88Sr, and the various trade-offs necessary to simultaneously trap 87Sr and 88Sr. Finally, I discuss how the thermalization studies of the scattering properties of the isotopes guide the forced evaporation of mixed isotope samples. These efforts result in the production of the BEC of 88Sr, but they also point the way to future studies of fermionic quantum degeneracy in 87Sr and to the rich physics of mixed species ultracold atomic systems.
Within the scope of this thesis, comprehensive experimental studies on interspecies interactions between 87Rb and various isotopes of Yb at temperatures in the µK regime have been performed. The interactions between Yb and 87Rb manifested themselves through interspecies thermalization and - in the case of 174Yb and 87Rb - through phase separation. The experimental investigations described in this thesis were carried out within a research project, whose long term goal is the creation of ultracold YbRb molecules. The experimental apparatus is designed for combined cooling and trapping of gaseous Yb and Rb atomic samples to the µK and nK regime. Due to the particular magnetic properties of Rb and Yb, a unique combination of trapping potentials, which allows for independent manipulation of ultracold 87Rb and Yb atoms, was used for the described measurements. The combined trap setup consisted of a Ioffe-Pritchard-type magnetic trap (MT) for 87Rb and a bichromatic optical dipole trap (BIODT) for Yb. The main scientific result of this PhD work is the observation of phase separation in a thermal mixture of 174Yb and 87Rb in the µK regime and the detailed experimental study and theoretical analysis of this unexpected effect. Analysis of the experimental observations required accurate modeling of the trapping potentials in order to extract quantitative results on the interspecies interaction. These results can be related to the 174Yb 87Rb interspecies s-wave scattering length, using an approach, which not only describes mean field effects but also includes multiple-particle interactions. This analysis yields a value for the s-wave scattering length of a_(87-174)=4710 (+3970) (-1520) a0, which is extraordinarily large. Indeed, this is to our knowledge the largest interspecies scattering length that has been observed to date at zero magnetic field. In contrast to observations of phase separation in other systems that have been made so far, phase separation between 174Yb and 87Rb as described in this PhD work is observed in purely thermal and not quantum degenerate samples. In this thesis, the study of phase separation in a 174Yb87Rb mixture was supplemented by an investigation of elastic collision rates for several of the Yb isotopes. The most notable finding was that for the combination \Yb{170}87Rb the interspecies scattering length has a value of a_(87-174)=6.6 (+3.5) (-2.9)a0 and is thus extremely small. The results of our studies on interspecies interactions between 87Rb and Yb can be used for model calculations on the YbRb molecular ground state potential: E. Tiesinga and S. Maxwell from the atomic theory group at NIST determined a Lennard-Jones potential, which quantitatively reproduces the present experimental observations. This knowledge will significantly help to provide a strategy for the controlled transfer of excited state YbRb* molecules to a selected ground state.