Observation of Efimov Resonances in a Mixture with Extreme Mass Imbalance

We observe two consecutive heteronuclear Efimov resonances in an ultracold Li-Cs mixture by measuring three-body loss coefficients as a function of magnetic field near a Feshbach resonance. The first resonance is detected at a scattering length of $a_{-}^{(0)}=-320(10)a_0$, corresponding to $\sim 7(\sim 3)$ times the Li-Cs (Cs-Cs) van der Waals range. The second resonance appears at $5.8(1.0)a_{-}^{(0)}$, close to the unitarity-limited regime at the sample temperature of 450 nK. Indication of a third resonance is found in the atom loss spectra. The scaling of the resonance positions is close to the predicted universal scaling value of 4.9 for zero temperature. Deviations from universality might be caused by finite-range and temperature effects, as well as magnetic field–dependent Cs-Cs interactions.

Figure 1

Magnetic field–dependent atom loss measurements of the Li-Cs mixture at a temperature of 400 nK. The Cs atom number after a hold time of 400 ms (a) and 1 s (b) is illustrated. The inset in (a) is a zoom in to the region where a third loss feature can be seen. The Li atoms show resonances at consistent positions after a hold time of 1.2 s (c) and (d), when the initial Li atom number is reduced by a factor of 2 as compared to the measurements in (a) and (b). Each point is the mean of at least six independent measurements, and the error bars represent the standard error. The dashed vertical lines indicate the position of the Efimov resonances. The resonance positions are determined via a fit of Cs atoms with Gaussian profiles with linear background (red solid lines). The scattering length scale has been assigned via radio frequency association of universal dimers [30].

Figure 2

Three-body loss coefficient $L_3^{\text{LiCsCs}}$ plotted versus the inverse scattering length $1/a$. The blue diamonds show the mean of three $L_3^{\text{LiCsCs}}$ measurements, where the error bars are given by the standard error. The red solid lines show Gaussian profiles with linear background fitted to the data to determine the position of the two Efimov resonances. The grey area illustrates the systematic error of 80% for the absolute value of $L_3^{\text{LiCsCs}}$. The inset shows a zoom in to the region of the first excited Efimov resonance.