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arXiv:cond-mat/0510630 · Physical Review A

Feshbach spectroscopy of a K−Rb atomic mixture

Francesca Ferlaino, Chiara D’Errico, G. Roati, Matteo Zaccanti, M. Inguscio, Giovanni Carlo Modugno, Andrea Simoni

Published Apr 14, 2006• 161 citations
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Abstract

We perform extensive magnetic Feshbach spectroscopy of an ultracold mixture of fermionic $^{40}\mathrm{K}$ and bosonic $^{87}\mathrm{Rb}$ atoms. The magnetic-field locations of 13 interspecies resonances are used to construct a quantum collision model able to predict accurate collisional parameters for all $\mathrm{K}\text{\ensuremath{-}}\mathrm{Rb}$ isotopic pairs. In particular, we determine the interspecies $s$-wave singlet and triplet scattering lengths for the $^{40}\mathrm{K}\text{\ensuremath{-}}^{87}\mathrm{Rb}$ mixture as $(\ensuremath{-}108\ifmmode\pm\else\textpm\fi{}3){a}_{0}$ and $(\ensuremath{-}205\ifmmode\pm\else\textpm\fi{}5){a}_{0}$, respectively. We also predict accurate scattering lengths and the position of Feshbach resonances for the other $\mathrm{K}\text{\ensuremath{-}}\mathrm{Rb}$ isotopic pairs. We discuss the consequences of our results for current and future experiments with ultracold $\mathrm{K}\text{\ensuremath{-}}\mathrm{Rb}$ mixtures.

DOI
10.1103/physreva.73.040702

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We perform extensive magnetic Feshbach spectroscopy of an ultracold mixture of fermionic 40K^{40}\mathrm{K}40K and bosonic 87Rb^{87}\mathrm{Rb}87Rb atoms. The magnetic-field locations of 13 interspecies resonances are used to construct a quantum collision model able to predict accurate collisional parameters for all K\ensuremath-Rb\mathrm{K}\text{\ensuremath{-}}\mathrm{Rb}K\ensuremath-Rb isotopic pairs. In particular, we determine the interspecies sss-wave singlet and triplet scattering lengths for the 40K\ensuremath-87Rb^{40}\mathrm{K}\text{\ensuremath{-}}^{87}\mathrm{Rb}40K\ensuremath-87Rb mixture as (\ensuremath−108\ifmmode±\else\textpm\fi3)a0(\ensuremath{-}108\ifmmode\pm\else\textpm\fi{}3){a}_{0}(\ensuremath−108\ifmmode±\else\textpm\fi3)a0​ and (\ensuremath−205\ifmmode±\else\textpm\fi5)a0(\ensuremath{-}205\ifmmode\pm\else\textpm\fi{}5){a}_{0}(\ensuremath−205\ifmmode±\else\textpm\fi5)a0​, respectively. We also predict accurate scattering lengths and the position of Feshbach resonances for the other K\ensuremath-Rb\mathrm{K}\text{\ensuremath{-}}\mathrm{Rb}K\ensuremath-Rb isotopic pairs. We discuss the consequences of our results for current and future experiments with ultracold K\ensuremath-Rb\mathrm{K}\text{\ensuremath{-}}\mathrm{Rb}K\ensuremath-Rb mixtures.
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.
Three magnetic-field induced heteronuclear Feshbach resonances were identified in collisions between bosonic 87Rb^{87}\mathrm{R}\mathrm{b}87Rb and fermionic 40K^{40}\mathrm{K}40K atoms in their absolute ground states. Strong inelastic loss from an optically trapped mixture was observed at the resonance positions of 492, 512, and $543\ifmmode\pm\else\textpm\fi{}2\text{ }\text{ }\mathrm{G}$. The magnetic-field locations of these resonances place a tight constraint on the triplet and singlet cross-species scattering lengths, yielding $(\ensuremath{-}281\ifmmode\pm\else\textpm\fi{}15){a}_{0}$ and $(\ensuremath{-}54\ifmmode\pm\else\textpm\fi{}12){a}_{0}$, respectively. The width of the loss feature at 543 G is $3.7\ifmmode\pm\else\textpm\fi{}1.5\text{ }\text{ }\mathrm{G}$ wide; this broad Feshbach resonance should enable experimental control of the interspecies interactions.
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We determine the interspecies sss-wave triplet scattering length a3{a}_{3}a3​ for all K-Rb isotopic mixtures by measuring the elastic cross section for collisions between 41K^{\mathrm{41}}\mathrm{K}41K and 87Rb^{87}\mathrm{R}\mathrm{b}87Rb in different temperature regimes. The positive value ${a}_{3}=+{163}_{\ensuremath{-}12}^{+57}{a}_{0}$ ensures the stability against collapse of binary $^{41}\mathrm{K}\ensuremath{-}^{87}\mathrm{R}\mathrm{b}$ Bose-Einstein condensates. For the fermion-boson mixture $^{40}\mathrm{K}\ensuremath{-}^{87}\mathrm{R}\mathrm{b}$, we obtain a large and negative scattering length which implies an efficient sympathetic cooling of the fermionic species down to the degenerate regime.
This thesis reports the observation of interspecies Feshbach resonances in an ultracold mixture of Rb and Cs atoms. A versatile combined magnetic and optical potential has been designed and constructed which is capable of bringing both 87Rb^{87}\rm{Rb}87Rb and 133Cs^{133}\rm{Cs}133Cs to degeneracy, and reaching high phase-space density in 85Rb^{85}\rm{Rb}85Rb. High phase-space density mixtures are the first step required in the production of ultracold polar molecules, the topic of much current research. The apparatus capitalises on the efficient capture of atoms by a magnetic trap from a magneto-optical trap, and the efficient sympathetic cooling of Cs by Rb therein. Upon transfer to the crossed optical dipole trap condensates in excess of 1×1061\times10^{6}1×106 87Rb^{87}\rm{Rb}87Rb atoms and approximately 1×1051\times10^{5}1×105 133Cs^{133}\rm{Cs}133Cs atoms are produced after direct evaporation and gravito-magnetic tilting of the potential. The observation of six interspecies 87Rb^{87}\rm{Rb}87Rb-133Cs^{133}\rm{Cs}133Cs Feshbach resonances are reported, three of which had only been predicted theoretically, allowing testing and development of the theoretical model. Furthermore, the extrapolation of this model has predicted numerous Feshbach resonances between 85Rb^{85}\rm{Rb}85Rb and 133Cs^{133}\rm{Cs}133Cs, none of which have been experimentally observed prior to this work. The versatile nature of this apparatus is discussed, including the application of the current system to cooling of 85Rb^{85}\rm{Rb}85Rb. Initial experiments observed seven interspecies resonances, including a broad s-wave resonance at a magnetic field of (644±2)(644\pm2)(644±2) G which is in excellent agreement with the theoretical prediction. Further work has revealed that fourteen Feshbach resonances exist in the 0-700 G magnetic field range between 85Rb^{85}\rm{Rb}85Rb and 133Cs^{133}\rm{Cs}133Cs atoms in the ∣2,+2⟩\left|2,+2\right\rangle∣2,+2⟩ and ∣3,+3⟩\left|3,+3\right\rangle∣3,+3⟩ states, respectively. Several of these resonances would be ideal for magneto-association of RbCs molecules, prior to transfer to the rovibrational ground-state.