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arXiv:cond-mat/0412049 · Physical Review Letters

Observation of Feshbach Resonances in an Ultracold Gas of Cr52

J. Werner, Axel Griesmaier, S. Hensler, J. Stühler, Tilman Pfau, Andrea Simoni, Eite Tiesinga

Published May 9, 2005• 162 citations
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Abstract

We have observed Feshbach resonances in collisions between ultracold $^{52}\mathrm{Cr}$ atoms. This is the first observation of collisional Feshbach resonances in an atomic species with more than one valence electron. The zero nuclear spin of $^{52}\mathrm{Cr}$ and thus the absence of a Fermi-contact interaction leads to regularly spaced resonance sequences. By comparing resonance positions with multichannel scattering calculations we determine the $s$-wave scattering length of the lowest $^{2S+1}\ensuremath{\Sigma}_{g}^{+}$ potentials to be $112(14)\text{ }\text{ }{a}_{0}$, $58(6)\text{ }\text{ }{a}_{0}$, and $\ensuremath{-}7(20)\text{ }\text{ }{a}_{0}$ for $S=6$, 4, and 2, respectively, where ${a}_{0}=0.0529\text{ }\text{ }\mathrm{nm}$.

DOI
10.1103/physrevlett.94.183201

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Due to atomic interactions, a dilute gas Bose-Einstein condensate acquires a self- energy proportional to the two-body scattering length a. This self-energy strongly influences the stability, formation rate, size and shape, collective excitations, and other properties of a condensate. For a given atomic species at zero magnetic field, the scattering length is fixed, and this had led to a rather limited range of interaction strengths in experiments. However, it has been predicted that a magnetically tunable Feshbach resonance could be used to strongly modify scattering lengths [1]. We have observed such a Feshbach resonance in collisions of ultracold 85Rb(f =2, mf=−2) atoms in a magnetic field B. From our measured resonance position and width, we infer that the scattering length a2,−2 for collisions of these atoms has the dispersive resonance form.
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 $\mathrm{K}\text{\ensuremath{-}}\mathrm{Rb}$ isotopic pairs. In particular, we determine the interspecies sss-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.
Magnetic Feshbach resonances play a central role in experimental research on atomic gases at ultracold temperatures, as they allow one to control the microscopic interactions between ultracold atoms by tuning an applied magnetic field. These resonances arise due to strong hyperfine interactions between the unpaired electron and the nuclear magnetic moment of the alkali metal atoms. A major thrust of current research is to create an ultracold gas of diatomic alkali metal molecules in the ground rovibrational state of the ground electronic ${}^{1}\ensuremath{\Sigma}$ state. Unlike alkali metal atoms, ${}^{1}\ensuremath{\Sigma}$ diatomic molecules have no unpaired electrons. However, the hyperfine interactions of molecules may give rise to magnetic Feshbach resonances. We use quantum scattering calculations to study the possible width of these resonances. Our results show that the widths of magnetic Feshbach resonances in ultracold molecule-molecule collisions for ${}^{1}\ensuremath{\Sigma}$ molecules may exceed 1 mG, rendering such resonances experimentally detectable. We hope that this work will stimulate the experimental search of these resonances.
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