Creation of ultracold heteronuclear p-wave Feshbach molecules

We report the creation of optically trapped ultracold heteronuclear p-wave Feshbach molecules in a mixture of 23Na and 87Rb atoms. With loss spectroscopy and binding energy measurements, we systematically characterize the interspecies p-wave Feshbach resonances near 284 G. Leveraging this understanding, we use magneto-association to form p-wave NaRb Feshbach molecules, producing both pure samples and mixtures of molecules in different angular momentum states. Additionally, we investigate the inelastic loss of these molecules, primarily influenced by atom-molecule and molecule-molecule collisions. Our results represent a significant step toward realizing tunable p-wave interactions in heteronuclear ultracold systems and provide a foundation for exploring non-zero angular momentum molecules.

💡 Research Summary

In this work the authors report the creation and comprehensive characterization of ultracold heteronuclear p‑wave Feshbach molecules formed from a mixture of ^23Na and ^87Rb atoms. The study begins with the identification of two closely spaced p‑wave interspecies Feshbach resonances near 284 G, each comprising two magnetic sub‑levels (m_F = 2 and m_F = 1 manifolds). By preparing the atomic mixture in the |F = 1, m_F = 1⟩ hyperfine state and varying the temperature from a few microkelvin down to sub‑microkelvin regimes, the authors record loss spectra that reveal strong temperature dependence: at low temperature the resonances narrow and split clearly, while at higher temperatures the peaks merge due to the centrifugal barrier’s influence on non‑s‑wave collisions.

To quantify the molecular binding energies, a magnetic‑field modulation technique is employed. The modulation frequency is swept while monitoring atom loss and cloud size, and the resulting asymmetric loss features are fitted with a Gaussian convolved with a Boltzmann distribution. This yields precise binding energies for each resonance and, importantly, a linear dependence of binding energy on magnetic field. The slopes correspond to the differential magnetic moments (δµ) between the open and closed channels. Measured values (≈ 980 kHz/G for (2, −1 & 0), 914 kHz/G for (2, 1), 2628 kHz/G for (1, 1), and 2887 kHz/G for (1, 0)) agree well with coupled‑channel calculations, confirming that the closed‑channel fraction remains essentially constant across the explored field range.

Magneto‑association (MA) is then used to convert atom pairs into p‑wave NaRb molecules. The stronger coupling of the m_F = 2 manifold enables efficient association despite the small 40 mG splitting between its two sub‑resonances. By sweeping the magnetic field at 0.7 G/ms from above 285 G down to 283.9 G, up to 1.2 × 10⁴ molecules are produced, corresponding to a conversion efficiency of about 6 % of the initial Rb atoms. The resulting molecular sample contains a mixture of the (2, −1 & 0) and (2, 1) m_ℓ states. A rapid magnetic‑field quench to 279.23 G, followed by a strong magnetic‑field gradient pulse, spatially separates the molecules from the remaining atoms, allowing detection via high‑field absorption imaging. Both magneto‑dissociation (MD) and photo‑dissociation (PD) are employed; MD offers state‑selective dissociation, whereas PD provides a more direct but less selective signal.

The authors investigate inelastic loss mechanisms by measuring molecular lifetimes in two scenarios. When residual atoms are present, the lifetime is only 0.8 ms, indicating rapid atom‑molecule collisions. After removing all Na and Rb atoms using microwave and blast pulses, a pure molecular sample exhibits a significantly longer lifetime of 12 ms, limited primarily by molecule‑molecule collisions and possibly by heating from the optical trap. By applying additional microwave pulses to transfer population into the (2, 1) state exclusively, they obtain a pure (2, 1) sample with a lifetime of 23.5 ms, suggesting that the remaining loss is dominated by light‑induced excitation rather than collisional processes.

Overall, the paper demonstrates for the first time the bulk production of heteronuclear p‑wave Feshbach molecules in a Bose‑Bose mixture, provides a detailed mapping of resonance properties (temperature shifts, magnetic‑moment differences, binding‑energy linearity), and characterizes loss channels that dictate molecular stability. These results open a pathway toward tunable anisotropic interactions in mixed‑species systems, enabling exploration of exotic many‑body phases such as finite‑momentum superfluids, topological p‑wave paired states, and controlled transfer of molecules to the rovibrational ground state via stimulated Raman processes. The ability to generate and manipulate p‑wave molecules in bulk, rather than in isolated tweezers, represents a significant advance for quantum simulation of non‑s‑wave physics.