Observation of Heteronuclear Atomic Efimov Resonances

Building on the recent experimental observation with ultracold atoms, we report the first experimental evidence of Efimov physics in a heteronuclear system. A mixture of ${}^{41}\mathrm{K}$ and ${}^{87}\mathrm{Rb}$ atoms was cooled to few hundred nanokelvins and stored in an optical dipole trap. Exploiting a broad interspecies Feshbach resonance, the losses due to three-body collisions were studied as a function of the interspecies scattering length. We observe an enhancement of the three-body collisions for three distinct values of the interspecies scattering lengths, both positive and negative, where no Feshbach resonances are expected. We attribute the two features at negative scattering length to the existence of two kinds of Efimov trimers, KKRb and KRbRb.

Figure 1

Pictorial energy diagram of Efimov states for the double-species mixture of K and Rb, around an interspecies Feshbach resonance where the K-Rb scattering length diverges ($1/a=0$). The Efimov resonances appear: (i) at the atom-dimer threshold for positive scattering lengths $a_{*}$; (ii) at the three-atoms threshold for negative scattering lengths $a_{-}$. Two distinct kinds of Efimov trimer are possible, KKRb (red or dark gray line) and KRbRb (blue or gray). The green or light gray line shows the dissociation threshold of the Efimov states.

Figure 2

Decay of the number of atoms trapped in our crossed dipole trap at $B=56.8\mathrm{G}$. At this magnetic field only collisions in KRbRb channel are relevant: the collision rate ${\alpha }_{\mathrm{KRbRb}}$ equals $1.9(7)\times {10}^{-23}{\mathrm{cm}}^6/\mathrm{s}$. The lines show the results of our numerical model including only KRbRb 3BR collisions. Hereafter, the error bars represent 1 standard deviation.

Figure 3

Peaks of inelastic atomic losses at 38.8 G (a),(b) and 57.7 G (c),(d), signaling the Efimov resonances at negative scattering lengths in the KKRb and KRbRb channel, respectively. We show the total number of atoms $N_{\mathrm{K}}+N_{\mathrm{Rb}}$ remaining in the optical trap after a fixed hold time at the given scattering length (a),(c). The hold times and temperatures are: 100 ms and $0.3\mu \mathrm{K}$ (a), 500 ms and $0.4\mu \mathrm{K}$ (c). The solid lines refer to the numerical results of our model. The dashed lines (a),(b) show the result of numerical integration taking into account only KRbRb collisions. In the insets (b) and (d) we plot the linear combinations $2N_{\mathrm{K}}-N_{\mathrm{Rb}}$ and $2N_{\mathrm{Rb}}-N_{\mathrm{K}}$, respectively, supporting the channel assignment of the Efimov peaks (see text).

Figure 4

Peak of enhanced inelastic atom losses at positive scattering lengths, corresponding to a possible atom-dimer resonance at 4.2 G. We show the total number of atoms $N_{\mathrm{K}}+N_{\mathrm{Rb}}$ remaining in the optical trap after a fixed hold time of 500 ms at the given scattering length. These data were taken at a temperature of 400 nK, each point averages on 2 to 5 experimental runs. The solid line is a Gaussian fit.