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arXiv:1701.02085 · Physical Review A

Investigation of Feshbach resonances in ultracold K40 spin mixtures

J. S. Krauser, Jannes Heinze, Sören Götze, Maria Langbecker, Nick Fläschner, L.G. Cook, Thomas M. Hanna, Eite Tiesinga, K. Sengstock, C. Becker

Published Apr 3, 2017• 16 citations
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Abstract

Magnetically tunable Feshbach resonances are an indispensable tool for experiments with atomic quantum gases. We report on 37 thus far unpublished Feshbach resonances and four further probable Feshbach resonances in spin mixtures of ultracold fermionic $^{40}\mathrm{K}$ with temperatures well below 100 nK. In particular, we locate a broad resonance at $B=389.7\phantom{\rule{0.16em}{0ex}}\text{G}$ with a magnetic width of $26.7\phantom{\rule{0.28em}{0ex}}\text{G}$. Here $1\phantom{\rule{0.28em}{0ex}}\text{G}={10}^{\ensuremath{-}4}\phantom{\rule{0.16em}{0ex}}\text{T}$. Furthermore, by exciting low-energy spin waves, we demonstrate a means to precisely determine the zero crossing of the scattering length for this broad Feshbach resonance. Our findings allow for further tunability in experiments with ultracold $^{40}\mathrm{K}$ quantum gases.

DOI
10.1103/physreva.95.042701

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Magnetically tunable Feshbach resonances are an indispensable tool for experiments with atomic quantum gases. We report on 37 thus far unpublished Feshbach resonances and four further probable Feshbach resonances in spin mixtures of ultracold fermionic 40K^{40}\mathrm{K}40K with temperatures well below 100 nK. In particular, we locate a broad resonance at B=389.7GB=389.7\phantom{\rule{0.16em}{0ex}}\text{G}B=389.7G with a magnetic width of 26.7G26.7\phantom{\rule{0.28em}{0ex}}\text{G}26.7G. Here 1G=10\ensuremath−4T1\phantom{\rule{0.28em}{0ex}}\text{G}={10}^{\ensuremath{-}4}\phantom{\rule{0.16em}{0ex}}\text{T}1G=10\ensuremath−4T. Furthermore, by exciting low-energy spin waves, we demonstrate a means to precisely determine the zero crossing of the scattering length for this broad Feshbach resonance. Our findings allow for further tunability in experiments with ultracold 40K^{40}\mathrm{K}40K quantum gases.
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.
We report the observation of a broad magnetic Feshbach resonance with a large background scattering length in an ultracold fermionic mixture of ^{23}Na^{40}K molecules and ^{40}K atoms, with both species prepared in their lowest hyperfine states. The Feshbach resonance is characterized by measuring resonantly enhanced loss rates and elastic scattering cross sections via cross-species thermalization. The large background scattering length can drive the atom-molecule mixture into the hydrodynamic regime when the magnetic field is far from the resonance. We observe that the center-of-mass motions of the atoms and molecules are phase locked and oscillate with a common frequency due to hydrodynamic drag effects. This broad atom-molecule Feshbach resonance with its large background scattering length opens up a new avenue toward studying strongly interacting fermionic gases with mass imbalance.
At the location of a magnetic-field Feshbach resonance, a mixture gas of fermionic atoms and dimers of fermionic atom pairs is investigated in the unitarity limit where the absolute value of the scattering length is much larger than the mean distance between atoms. The dynamic equilibrium of the mixture gases is characterized by the minimum of the Gibbs free energy. For the fermionic atoms and dimers with divergent scattering length, it is found that the fraction of the dimers based on a very simple theory agrees with the high fraction of zero-momentum molecules observed in a recent experiment (M. W. Zwierlein et al, Phys. Rev. Lett. 92, 120403 (2004)). The dimeric gas can be also used to interpret the frequency of the radial breathing mode observed in the experiment by J. Kinast et al (Phys. Rev. Lett. 92, 150402 (2004)).
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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.
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.