We study the widths of interspecies Feshbach resonances in a mixture of the fermionic quantum gases 6Li and 40K. We develop a model to calculate the width and position of all available Feshbach resonances for a system. Using the model, we select the optimal resonance to study the {6}Li/{40}K mixture. Experimentally, we obtain the asymmetric Fano line shape of the interspecies elastic cross section by measuring the distillation rate of 6Li atoms from a potassium-rich 6Li/{40}K mixture as a function of magnetic field. This provides us with the first experimental determination of the width of a resonance in this mixture, DeltaB=1.5(5) G. Our results offer good perspectives for the observation of universal crossover physics using this mass-imbalanced fermionic mixture.

We present an asymptotic-bound-state model which can be used to accurately describe all Feshbach resonance positions and widths in a two-body system. With this model we determine the coupled bound states of a particular two-body system. The model is based on analytic properties of the two-body Hamiltonian and on asymptotic properties of uncoupled bound states in the interaction potentials. In its most simple version, the only necessary parameters are the least bound state energies and actual potentials are not used. The complexity of the model can be stepwise increased by introducing threshold effects, multiple vibrational levels, and additional potential parameters. The model is extensively tested on the 6 Li-40 K system and additional calculations on the 40 K-87 Rb system are presented.

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We discuss the stability of homonuclear and heteronuclear mixtures of 3 He and 4 He atoms in the metastable 2 3 S 1 state (He * ) and predict positions and widths of Feshbach resonances by using the asymptotic-bound-state model. All calculations are performed without fit parameters, using ab initio calculations of molecular potentials. One promising very broad Feshbach resonance ( B = 72.9 +18.3 −19.3 mT) is found that allows for tuning of the interisotope scattering length.

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