Observation of interspecies Feshbach resonances in an ultracold Rb-Cs mixture

We report on the observation of interspecies Feshbach resonances in an ultracold, optically trapped mixture of Rb and Cs atoms. In a magnetic-field range up to 300 G we find 23 interspecies Feshbach resonances in the lowest spin channel and two resonances in a higher channel of the mixture. The extraordinarily rich Feshbach spectrum suggests the importance of different partial waves in both the open and closed channels of the scattering problem along with higher-order coupling mechanisms. Our results, on one hand, provide fundamental experimental input to characterize the Rb-Cs scattering properties and, on the other hand, identify possible starting points for the association of ultracold heteronuclear RbCs molecules.

Figure 1

(Color online) Illustration of the two-color DRSC technique applied simultaneously to the Rb (blue solid lines) and Cs (red dashed lines) atoms. For each species, a cycling transition consists of a two-photon Raman transition (indicated by the curved arrows) between vibrational levels of $m_F$-dependent harmonic microtraps followed by optical pumping to the lowest Zeeman sublevel (not shown). Rb and Cs atoms accumulate in the vibrational ground state of the $m_F=1$ and $m_F=3$ levels, respectively, which, as dark states, are essentially decoupled from the light field.

Figure 2

(Color online) Observation of heteronuclear Rb-Cs Feshbach resonances by loss spectroscopy. The spectrum is recorded for Cs atoms, being the minority component of the mixture. The remaining fraction represents the number of Cs atoms after a 5 s hold time in the dipole trap normalized to the corresponding average number detected off any resonance. The arrows indicate the positions of the interspecies Feshbach resonances. Other loss features correspond to homonuclear Cs Feshbach resonances. (a) shows the loss spectrum for the $\text{Rb}|1,1⟩+\text{Cs}|3,3⟩$ channel. (b) displays the $\text{Rb}|2,-1⟩+\text{Cs}|3,3⟩$ channel. Inset illustrates the transfer scheme applied to populate the $\text{Rb}|2,-1⟩$ state.

Figure 3

(Color online) Example of a Rb-Cs resonance pair observed in the $\text{Rb}|1,1⟩+\text{Cs}|3,3⟩$ channel. The circles represent a subset of the data shown in Fig. 2 and the solid line is a fit to the data using a double Lorentzian profile. The resonance splitting is about 1.3 G.