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arXiv:0705.2750 · Physical Review A

Ultracold thermalization of Li7 and Rb87

C. Marzok, Benjamin Deh, Ph. W. Courteille, Claus Zimmermann

Published Nov 8, 2007• 20 citations
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Abstract

We report on measurements of cross-species thermalization inside a magnetically trapped spin-polarized mixture of $^{87}\mathrm{Rb}$ and $^{7}\mathrm{Li}$ atoms with both atoms in their respective low-field-seeking magnetic substates $\ensuremath{\mid}F=2,{m}_{F}=2⟩$. Measurement of the thermalization velocity in the ultracold regime below $10\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{K}$ allows for the derivation of the absolute value of the pure triplet $s$-wave scattering length governing the interaction. We find $\ensuremath{\mid}{a}_{7,87}\ensuremath{\mid}=(59\ifmmode\pm\else\textpm\fi{}19){a}_{\mathrm{B}}$. We propose to study both species in the condensed regime to derive the sign of ${a}_{7,87}$. In this context, we present numerical solutions to the coupled Gross-Pitaevskii equation based on the hypothesis of a positive sign. According to the simulations, phase separation of the Li and Rb $\ensuremath{\mid}2,2⟩$ clouds occurs along with a mean-field stabilization allowing for larger atom numbers of condensed $^{7}\mathrm{Li}$ atoms before collapse sets in. Observation of this mean-field stabilization would directly fix the sign of ${a}_{7,87}$. We discuss our results in the light of this proposal.

DOI
10.1103/physreva.76.052704

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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.
We report on the observation of five Feshbach resonances in collisions between ultracold 7Li^{7}\mathrm{Li}7Li and 87Rb^{87}\mathrm{Rb}87Rb atoms in the absolute ground-state mixture where both species are in their $\ensuremath{\mid}f,{m}_{f}⟩=\ensuremath{\mid}1,1⟩$ hyperfine states. The resonances appear as trap losses for the 7Li^{7}\mathrm{Li}7Li cloud induced by inelastic heteronuclear three-body collisions. The magnetic field values where they occur are important quantities for an accurate determination of the interspecies interaction potentials. Results of coupled channels calculations based on the observed resonances are presented and refined potential parameters are given. A very broad Feshbach resonance centered around 649G649\phantom{\rule{0.3em}{0ex}}\mathrm{G}649G should allow for fine-tuning of the interaction strength in future experiments.
We report on the observation of two Feshbach resonances in collisions between ultracold 6Li^{6}\mathrm{Li}6Li and 87Rb^{87}\mathrm{Rb}87Rb atoms in their respective hyperfine ground states $\ensuremath{\mid}F,{m}_{F}⟩=\ensuremath{\mid}1∕2,1∕2⟩$ and \ensuremath{\mid}1,1⟩. The resonances show up as trap losses for the 6Li^{6}\mathrm{Li}6Li cloud induced by inelastic Li-Rb-Rb three-body collisions. The magnetic field values where they occur represent important benchmarks for an accurate determination of the interspecies interaction potentials. A broad Feshbach resonance located at 1066.92G1066.92\phantom{\rule{0.3em}{0ex}}\mathrm{G}1066.92G opens interesting prospects for the creation of ultracold heteronuclear molecules. We furthermore observe a strong enhancement of the narrow ppp-wave Feshbach resonance in collisions of 6Li^{6}\mathrm{Li}6Li atoms at 158.55G158.55\phantom{\rule{0.3em}{0ex}}\mathrm{G}158.55G in the presence of a dense 87Rb^{87}\mathrm{Rb}87Rb cloud. The effect of the 87Rb^{87}\mathrm{Rb}87Rb cloud is to introduce Li-Li-Rb three-body collisions occurring at a higher rate than Li-Li-Li collisions.
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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.