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arXiv:2107.11269 · Physical Review Research

Strong spin-exchange recombination of three weakly interacting Li7 atoms

Jing-Lun Li, T. Secker, P. M. A. Mestrom, S. J. J. M. F. Kokkelmans

Published May 9, 2022• 14 citations
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Abstract

Quantifying the nonuniversal three-body losses in ultracold atomic gases has been a long-standing problem. We go beyond earlier approaches to solve this problem by using a model that includes realistic pairwise interaction potentials and the exact three-atom spin structure. With this model we can successfully explain experimental observations for weakly interacting $^{7}\mathrm{Li}$ and $^{87}\mathrm{Rb}$ atoms. Capturing the exact spin structure, we are able to reveal a strong three-body spin-exchange recombination mechanism. In addition, we predict a regime of low three-body recombination rate in the $^{7}\mathrm{Li}$ system that is advantageous for a variety of many-body experiments. This work opens avenues to study spin-involved few-body processes in quantum gases.

DOI
10.1103/physrevresearch.4.023103

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Quantifying the nonuniversal three-body losses in ultracold atomic gases has been a long-standing problem. We go beyond earlier approaches to solve this problem by using a model that includes realistic pairwise interaction potentials and the exact three-atom spin structure. With this model we can successfully explain experimental observations for weakly interacting 7Li^{7}\mathrm{Li}7Li and 87Rb^{87}\mathrm{Rb}87Rb atoms. Capturing the exact spin structure, we are able to reveal a strong three-body spin-exchange recombination mechanism. In addition, we predict a regime of low three-body recombination rate in the 7Li^{7}\mathrm{Li}7Li system that is advantageous for a variety of many-body experiments. This work opens avenues to study spin-involved few-body processes in quantum gases.
We report on measurements of three-body recombination loss rates in an ultracold gas of ^{7}Li atoms in the extremely nonuniversal regime where the two-body scattering length vanishes. We show that the loss rate coefficient is well defined and can be described by two-body parameters only: the scattering length a and the effective range R_{e}. We find the rate to be energy independent, and, by connecting our results with previously reported measurements in the universal limit, we cover the behavior of the three-body recombination rate in the whole range from weak to strong two-body interactions. We identify a nontrivial magnetic field value in the nonuniversal regime where the rate should be suppressed.
We resolve the unexpected and long-standing disagreement between experiment and theory in the Efimovian three-body spectrum of ^{7}Li, commonly referred to as the lithium few-body puzzle. Our results show that the discrepancy arises out of the presence of strong nonuniversal three-body spin-exchange interactions, which enact an effective inflation of the universal Efimov spectrum. This conclusion is obtained from a thorough numerical solution of the quantum mechanical three-body problem, including precise interatomic interactions and all spin degrees of freedom for three alkali-metal atoms. Our results show excellent agreement with the experimental data regarding both the Efimov spectrum and the absolute rate constants of three-body recombination, and in addition reveal a general product propensity for such triatomic reactions in the Paschen-Back regime, stemming from Wigner's spin conservation rule.
We develop a full multichannel spin model in momentum space to investigate three-body recombination of identical alkali-metal atoms colliding in a magnetic field. The model combines the exact three-atom spin structure and realistic pairwise atom-atom interactions. By neglecting the interaction between two particles when the spectating particle is not in its initial spin state we arrive at an approximate model. With this approximate model we achieve excellent agreement with the recent precise measurement of the ground Efimov resonance position in potassium-39 close to 33.58 G [R. Chapurin et al., Phys. Rev. Lett. 123, 233402 (2019)]. We analyze the limitations of our approximation by comparing to the numerical results for the full system and find that it breaks down for Feshbach resonances at larger magnetic fields in the same spin channel. There the relevant three-body closed channel thresholds are much closer to the open channel threshold, which enhances the corresponding multichannel couplings. Therefore the neglected components of the interaction should be included for those Feshbach resonances.
In view of the failure of the Kagan dipole mechanism to explain the magnetic field dependence of the H+H+H recombination rate in spin-polarized atomic hydrogen, we consider an additional process, the so-called dipole-exchange mechanism. Two simple approaches to estimate its consequences turn out to be promising but the question of the role of different and higher-order processes remains open. We therefore turn to in principle, an exact approach to the three-body recombination, including all possible processes. The first numerical results of the approach are presented.
We are developing theoretical and computational methods to study two related three-body processes in ultracold quantum gases: three-body resonances and three-body recombination. Three-body recombination causes the ultracold gas to heat up and atoms to leave the trap where they are confined. Therefore, it is an undesirable effect in the process of forming ultracold quantum gases. Metastable three-body states (resonances) are formed in the ultracold gas. When decaying they also give additional kinetic energy to the gas, that leads to the heating too. In addition, a reliable method to obtain three-body resonances would be useful in a number of problems in other fields of physics, for example, in models of metastable nuclei or to study dissociative recombination of H3 +. Our project consists of employing computer modeling to develop a method to obtain three-body resonances. The method uses a novel two-step diagonalization approach to solve the three-body Schrödinger equation. The approach employs the SVD method of Tolstikhin et al. coupled with a complex absorbing potential. We tested this method on a model system of three identical bosons with nucleon mass and compared it to the results of a previous study. This model can be employed to understand the 3He nucleus . We found one three-body bound state and four resonances. We are also studying Efimov resonances using a 4He-based model. In a system of identical spinless bosons, Efimov states are a series of loosely bound three-body states which begin to appear as the energy of the two-body bound state approaches zero . Although they were predicted 35 years ago, recent evidence of Efimov states found by Kraemer et al. in a gas of ultracold Cs atoms has sparked great interest by theorists and experimentalists. Efimov resonances are a kind of pre-dissociated Efimov trimer. To search for Efimov resonances we tune the diatom interaction potential, V(r): V(r) → λV(r) as Esry et al. did . We calculated the first two values of λ for which there is a "condensation" (infinite number) of Efimov states. They are λEfimov1 = 0.9765 and λEfimov2 = 6.834. We performed calculations for λ = 2.4, but found no evidence of Efimov resonances. For future work we plan to work with λ ≈ 4 and λ ≈ λEfimov2 where we might see d-wave and higher l-wave Efimov resonances. There is also a many-body project that forms part of this thesis and consists of a direct diagonalization of the Bogolyubov Hamiltonian, which describes elementary excitations of a gas of bosons interacting through a pairwise interaction. We would like to reproduce the corresponding energy spectrum. So far we have performed several convergence tests, but have not observed the desired energy spectrum. We show preliminary results.
We theoretically investigate the spin structure of weakly bound diatomic van der Waals molecules formed by two identical bosonic alkali atoms. Our studies were performed using known Born-Oppenheimer potentials while developing a reduced interaction potential model. Such reduced potential models are currently a key for solving certain classes of few-body problems of atoms as they decrease the numerical burden on the computation. Although the reduced potentials are significantly shallower than actual Born-Oppenheimer potentials, they still capture the main properties of the near-threshold bound states, including their spin structure, and the scattering states over a broad range of magnetic fields. At zero magnetic field, we find that the variation in spin structure across different alkali species originates from the interplay between electronic spin exchange and hyperfine interactions. To characterize this competition we introduce a single parameter that is a function of the singlet and triplet scattering lengths, the atomic hyperfine splitting constant, and the molecular binding energy. We show that this parameter can be used to classify the spin structure of vdW molecules for each atomic species.