Session Q09: Ultracold Collisions and Few-body Systems

Ion-atom-atom three-body recombination: from cold chemistry to plasma physics

This work presents a study on three-body recombination involving an ion and two atoms toward forming molecular ions and molecules in a wide range of energies and systems, from cold chemistry systems to plasma-relevant scenarios. In particular, we calculate the three-body recombination rate via classical trajectory calculations in hyperspherical coordinates. Our results show that, independently of the nature of the system at hand, there are two very distinct dynamical regimes. First, the low-energy regime dominated the long-range tail of the interaction potential, and the second one, the high-energy regime, dominated by the short-range details of the potential. The transition between these two regimes occurs when the collision energy is of the same order as the dissociation energy of the molecular ion. Thus, explaining why threshold laws are still applicable to room temperature.   

Spin propensity rules for three-body recombination of ultracold alkali-metal atoms

We present a theoretical study on the spin-propensity rule for the final state distribution of three-body recombination for all commonly used alkali-metal atoms. Three-body recombination is a chemical reaction in which three atoms collide to produce a diatomic molecule which can further detected in molecular ionization experiments. Our previous studies with Rubidium atoms have shown that most of the molecular states formed in recombination have the same hyperfine spins as those of the colliding atoms. Our current study, however, shows that the conservation of the hyperfine spins is dependent on the strength of the electronic spin-exchange. We find that various atomic species violate such conservation rule, while still displaying propensity rules related to the binding energy of the molecular products.   

Dynamically probing three-body systems in cold gases

Knowing the intrinsic aspects of few-body systems, such as binding energies, and lifetimes, is highly desirable since they affect the macroscopic properties of a gas. On a two-body level there are well-established dynamical protocols addressing their attributes, however, this is not the case for three-body systems. Our study focuses on the dynamics of three bosons triggered by a double sequence of magnetic field pulses. This Ramsey-like protocol permits the creation of trimers and by varying the dark time between the two pulses an oscillatory pattern emerges in the fraction of formed trimers. The spectra of these interference fringes provide simultaneously the binding energy of trimers as well as their lifetimes, and they are robust against thermal effects. This means that the proposed dynamical protocol can be potentially used to determine the intrinsic characteristics of exotic few-body states, e.g. Efimov states, regardless the sign or the magnitude of the scattering length.   

Multichannel effects and universality in three-body physics of strongly interacting 7Li

We study the apparent disagreement between experimental measurement of three-body observables in ultracold lithium-7 and the predictions of universal theory, which has shown to be generally accurate in describing the other alkali-metals. For this purpose, we apply a sophisticated numerical approach based on an expansion and subsequent diagonalisation of the full three-body Hamiltonian in momentum space. With this method, we are able to take into account both realistic molecular interactions with many low-lying dimer states, and the complete three-body hyperfine basis that underpins the multichannel nature of the problem. Our results show that multichannel effects in lithium-7 shift the three-body parameter away from the universal prediction and towards the current experimental data. By analysing partial recombination losses, we find that this sensitivity can be attributed to a strong coupling to specific three-body hyperfine channels, which have often been neglected in earlier studies. In addition, we apply our methods to the computation of the Efimov binding energy, and compare directly with state-of-the-art experiments.   

Chemical reactions of ultracold alkaline-earth-metal diatomic molecules

We study the energetics of chemical reactions between ultracold ground-state alkaline-earth-metal diatomic molecules. We show that the atom-exchange reactions forming homonuclear dimers are energetically allowed for all heteronuclear alkaline-earth-metal combinations. We perform high-level electronic structure calculations on the potential energy surfaces of all possible homo- and heteronuclear alkaline-earth-metal trimers and show that trimer formation is also energetically possible in collisions of all considered dimers. Interactions between alkaline-earth-metal diatomic molecules lead to the formation of deeply bound reaction complexes stabilized by large non-additive interactions. We check that there are no barriers to the studied chemical reactions. This means that all alkaline-earth-metal diatomic molecules are chemically unstable at ultralow temperature, and optical lattice or shielding schemes may be necessary to segregate the molecules and suppress losses. Our findings explain recent experimental observations of fast reactive trap losses between ultracold ground-state Sr₂ molecules.   

Temperature-dependent contact of weakly interacting single-component Fermi gases and loss rate of degenerate polar molecules

Motivated by the experimental realization of single-component degenerate Fermi gases of polar ground state KRb  molecules with intrinsic two-body losses [Science 363, 853 (2019)], this work studies the finite-temperature loss rate of single-component Fermi gases with weak interactions. First, we establish a relationship between the two-body loss rate and the p-wave contact. Second, we evaluate the contact of the homogeneous system in the low-temperature regime using p-wave Fermi liquid theory and in the high-temperature regime using the second-order virial expansion. Third, conjecturing that there are no phase transitions between the two temperature regimes, we smoothly interpolate the results to intermediate temperatures. It is found that the contact is constant at temperatures close to zero and increases first quadratically with increasing temperature and finally---in agreement with the Bethe-Wigner threshold law---linearly at high temperatures. Fourth, applying the local-density approximation, we obtain the loss rate for the harmonically trapped system, reproducing the experimental KRb loss measurements within a unified theoretical framework over a wide temperature regime without fitting parameters. Our results for the contact are not only applicable to molecular p-wave gases but also to atomic single-component Fermi gases, such as ⁴⁰K and ⁶Li.   

Evidence of Quantum Chaos in Feshbach Resonances of Ultracold 87Rb+23Na87Rb

We report the observation of magnetically tunable Feshbach resonances between ultracold ⁸⁷Rb atoms and ground-state ²³Na⁸⁷Rb molecules. More than 50 resonances have been observed in each of two different collisional channels in the magnetic field range of about 500 Gauss. Analyzing the distributions of the resonances statistically and comparing them to models based on random matrix theory, we obtain the first evidence of chaotic scattering in the atom-molecule system.   

Stereodynamical control of cold collisions between two aligned D2 molecules

Resonant scattering of optically state-prepared and aligned molecules in the cold regime allows the most detailed interrogation and control of bimolecular collisions. This technique has recently been applied to collisions of two aligned ortho-D₂ molecules prepared in the j=2 rotational level of the v=2 vibrational manifold using the Stark-induced adiabatic Raman passage technique. Here, we develop the theoretical formalism for describing four-vector correlations in collisions of two aligned molecules and apply our approach to state-prepared D₂(v=2,j=2)+ D₂(v=2,j=2)→D₂(v=2,j=2)+ D₂(v=2,j=0) collisions, making possible the simulations of the experimental results from first principles. Key features of the experimental angular distributions are reproduced and attributed primarily to a partial wave resonance with orbital angular momentum l=4.   

d-wave scattering resonances in a 39K Bose gas

The tunability of interactions in cold atom systems has proven to be one of the most powerful tools in the field. Most success stories on exploring different interaction regimes, including that of unitarity, so far relied on utilising s-wave Feshbach resonances. Exploitation of higher-partial-wave scattering resonances can open even more possibilities, such as studies of exotic superfluids, but these (generally narrower) resonances remain far less explored. We perform a systematic experimental study of d-wave scattering resonances in the F=1 hyperfine manifold of ³⁹K using atom-loss spectroscopy. Our measurements of resonances in different spin states, and at different gas densities and temperatures, will help identify most promising routes for future many-body experiments.     

Fragmented condensate state in a spin-1/2 Bose gas with p-wave interaction

In this talk, I show that a collection of spin-1/2 bosons subjected to p-wave interaction in addition to the usual s-wave interaction can, under certain conditions, develop a novel condensate state in which two bosons are correlated in a spin singlet states with a corresponding p-wave orbital wave function that helps to lower the energy of the system. In addition, we also explored the sound modes of the system and show how it is affected by the p-wave interaction.