Feshbach spectroscopy of an ultracold mixture of ${}^{85}$Rb and ${}^{133}$Cs

We report the observation of interspecies Feshbach resonances in an optically trapped mixture of ${}^{85}$Rb and ${}^{133}$Cs. We measure nine resonances in the lowest spin channel for a magnetic field range from 0 to 700 G and show that they are in good agreement with coupled-channel calculations. The interspecies background scattering length is close to zero over a large range of magnetic fields, permitting the sensitive detection of Feshbach resonances through interspecies thermalization. Our results confirm the quality of the Rb-Cs potential curves [Phys. Rev. A 85, 032506 (2012)] and offer promising starting points for the production of ultracold polar molecules.

Figure 1

Top: Observation of interspecies Feshbach resonances by thermalization of the two species for fields up to 700 G. The closed (open) symbols indicate the ${}^{85}$Rb (${}^{133}$Cs) temperature while solid (dashed) arrows mark observed $s$-wave ($p$-wave) resonances and dotted arrows mark laser-induced features. Error bars show the standard deviations for multiple control shots at specific magnetic fields. Bottom: The calculated $s$-wave interspecies scattering length and weakly bound molecular states for the same field range. Bound states arising from $L=0$ ($s$ states) are shown as black lines while those from $L=2$ ($d$ states) are shown in other colors that indicate the value of $M_F$ (see figure). The bound-state energies are plotted relative to the energy of the lowest hyperfine state of ${}^{85}$Rb + ${}^{133}$Cs, the $|2,+2\rangle$+$|3,+3\rangle$ hyperfine level. Resonance positions are marked on the scattering length plot using vertical lines of the same color as the corresponding bound state; the length of each line is proportional to the logarithm of the width of the resonance. The gray shaded region indicates where $\left|a\right|<60$ bohr. All the bound states shown are for $M_{\mathrm{tot}}=5$, corresponding to $s$-wave scattering in the lowest channel.

Figure 2

An interspecies resonance observed at 187.66(5) G. The closed (open) symbols indicate ${}^{85}$Rb (${}^{133}$Cs) data points. (a) Temperature data for both species reveal the thermalization of ${}^{133}$Cs with ${}^{85}$Rb at and near the resonance highlighting the tunable nature of the sympathetic cooling. (b) Atom number data show the ${}^{133}$Cs number to increase while the ${}^{85}$Rb number decreases when a resonance is crossed and sympathetic cooling occurs. Error bars show the standard deviation for multiple control shots at a specific magnetic field.

Figure 3

An intraspecies ${}^{85}$Rb resonance at 368.78(3) G neighboring an interspecies resonance at 370.39(1) G. Closed (open) symbols indicate ${}^{85}$Rb (${}^{133}$Cs) data. (a) Loss of ${}^{85}$Rb reveals both resonances. (b) Temperature data allow the resonances to be distinguished as intra- or interspecies. The ${}^{133}$Cs temperature is unchanged at the ${}^{85}$Rb intraspecies resonance but decreases at the interspecies resonance at higher field where interspecies collisions cause sympathetic cooling. Error bars show the standard deviation for multiple control shots at a specific magnetic field.