Universal Three-Body Physics in Ultracold KRb Mixtures

Ultracold atomic gases have recently become a driving force in few-body physics due to the observation of the Efimov effect. While initially observed in equal mass systems, one expects even richer few-body physics in the heteronuclear case. In previous experiments with ultracold mixtures of potassium and rubidium, an unexpected nonuniversal behavior of Efimov resonances was observed. In contrast, we measure the scattering length dependent three-body recombination coefficient in ultracold heteronuclear mixtures of ${}^{39}\mathrm{K}\text{−}{}^{87}\mathrm{Rb}$ and ${}^{41}\mathrm{K}\text{−}{}^{87}\mathrm{Rb}$ and do not observe any signatures of Efimov resonances for accessible scattering lengths in either mixture. Our results show good agreement with our theoretical model for the scattering dependent three-body recombination coefficient and reestablish universality across isotopic mixtures.

Figure 1

Atom number and temperature for a measurement of the three-body recombination coefficient. Top panel: Number of ${}^{87}\mathrm{Rb}$ (solid red circles) and ${}^{39}\mathrm{K}$ (open blue circles) atoms. Bottom panel: Weighted average of the two cloud temperatures (black diamonds). The lines correspond to the fitted numerical solutions of the differential equations (see the text).

Figure 2

Three-body recombination coefficient as a function of scattering length for the ${}^{39}\mathrm{K}\text{−}{}^{87}\mathrm{Rb}$ mixture. The solid blue line at negative scattering lengths is obtained from our theoretical model. For positive scattering lengths, a fit with an $a^4$ dependence (blue line) and possible Efimov recombination minima at $2800a_0$ (dashed yellow line) and $5100a_0$ (dash-dotted green line) are displayed (see the text). The error bars represent weighted averages of the fit uncertainties.

Figure 3

Three-body recombination coefficient as a function of scattering length for the ${}^{41}\mathrm{K}\text{−}{}^{87}\mathrm{Rb}$ mixture recorded using two Feshbach resonances at 38 G (solid circles) and 79 G (open circles). At negative scattering lengths, our theoretical model is used to display three different scenarios: no Efimov resonance (solid blue line), a resonance at $-246a_0$ (dashed red line), and a resonance at $-450a_0$ (dash-dotted purple line). For positive scattering lengths, a fit with an $a^4$ dependence (blue line) and possible Efimov recombination minima at $2800a_0$ (dashed yellow line) and $5100a_0$ (dash-dotted green line) are displayed (see the text). The error bars represent weighted averages of the fit uncertainties.

Figure 4

Collisional spectroscopy of a $p$-wave Feshbach resonance. Summed atom number of ${}^{41}\mathrm{K}$ and ${}^{87}\mathrm{Rb}$ remaining in the trap for a fixed hold time as a function of magnetic field. The solid line is a fit of two Gaussians on a linear background. A typical error bar is displayed.