Universal three-body recombination and Efimov resonances in an ultracold Li-Cs mixture

We study Efimov resonances via three-body loss in an ultracold two-component gas of fermionic ${}^6\mathrm{Li}$ and bosonic ${}^{133}\mathrm{Cs}$ atoms close to a Feshbach resonance at 843 G, extending results reported previously [Pires et al., Phys. Rev. Lett. 112, 250404 (2014)] to temperatures around 120 nK. The experimental scheme for reaching lower temperatures is based upon compensating the gravity-induced spatial separation of the mass-imbalanced gases with bichromatic optical dipole traps. We observe the first and second excited Li-Cs-Cs Efimov resonance in the magnetic field dependence of the three-body event rate constant, in good agreement with the universal zero-range theory at finite temperature [Petrov and Werner, Phys. Rev. A 92, 022704 (2015)]. Deviations are found for the Efimov ground state, and the inelasticity parameter $\eta$ is found to be significantly larger than those for single-species systems.

Figure 1

(a) Top view of the dimple and tunable traps in the horizontal plane (gravity points into the plane). (b)–(g) Modeled optical dipole plus gravitational potentials for Li (red, solid) and Cs (blue, dashed) atoms at 100 and 400 nK and their corresponding atomic density distributions (Li and Cs, shaded red and hatched blue areas, respectively). (b) and (e) In the horizontal direction the clouds always overlap independent of laser intensity. (c) and (f) Using solely the dimple trap the spatial overlap is already reduced to 80% at 400 nK, while at 100 nK Cs atoms can no longer be stored due to the influence of the gravitational potential. (d) and (g) Combined trap. At 100 nK the intensity is $7.0\times {10}^3$ mW/${\mathrm{mm}}^2$ for the dimple trap and $6.3\times {10}^3$ mW/${\mathrm{mm}}^2$ for the tunable trap.

Figure 2

Temporal evolution of a Li-Cs mixture at 843.049 G and 120 nK. (a) Red solid (Li) and blue dashed (Cs) lines show the result of a simultaneous fit of the coupled rate equations [(1) and (2)] to the atom number of Li (red circles) and Cs (blue diamonds). (b) and (c) Respective $1/e$ widths of the Li and Cs atom clouds in two directions. Each data point gives the mean of at least three independent measurements and the error bars represent one standard error. Symbols and color coding are the same as in (a).

Figure 3

Li-Cs-Cs three-body event rate constant $L_3$ at 450 nK (red) and at 120 nK (blue). Experimental data (red diamonds and blue squares, circles, triangles) have been scaled in the vertical direction using a fit to the zero-range theory (solid red lines and dashed blue lines). The shaded region around the fits represent the uncertainty of the Feshbach resonance position. Note that the maximum of $L_3$ does not correspond to the Feshbach resonance position (see Ref. [34]). The first and second excited Efimov states are clearly visible at $\approx 843.8$ G and $\approx 843.0$ G, respectively. Data beyond 845 G represent an upper bound of the loss coefficient at 120 nK due to additional loss dynamics. The inset shows that the Efimov ground state at $\approx 848.9$ G at 450 nK deviates from the zero-range theory. The 450 nK data are taken from our previous publication [32]. The pole of the Feshbach resonance is indicated by the dotted line and the uncertainty by the gray shade.