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arXiv:2210.15981 · Physical Review A

Anomalous loss behavior in a single-component Fermi gas close to a p -wave Feshbach resonance

Kilian Welz, Manuel Gerken, Bing Zhu, Eleonora Lippi, M. Rautenberg, Lauriane Chomaz, Matthias Weidemüller

Published May 11, 2023• 4 citations
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Abstract

We theoretically investigate three-body losses in a single-component Fermi gas near a $p$-wave Feshbach resonance in the interacting, nonunitary regime. We extend the cascade model introduced by Waseem et al. [M. Waseem, J. Yoshida, T. Saito, and T. Mukaiyama, Phys. Rev. A 99, 052704 (2019)] to describe the elastic and inelastic collision processes. We find that the loss behavior exhibits a ${n}^{3}$ and an anomalous ${n}^{2}$ density dependence for a ratio of elastic-to-inelastic collision rate larger and smaller than 1, respectively. The corresponding evolutions of the energy distribution show collisional cooling or evolution toward low-energetic nonthermalized steady states, respectively. These findings are particularly relevant for understanding atom loss and the energetic evolution of ultracold gases of fermionic lithium atoms in their ground state.

DOI
10.1103/physreva.107.053310

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We theoretically investigate three-body losses in a single-component Fermi gas near a ppp-wave Feshbach resonance in the interacting, nonunitary regime. We extend the cascade model introduced by Waseem et al. [M. Waseem, J. Yoshida, T. Saito, and T. Mukaiyama, Phys. Rev. A 99, 052704 (2019)] to describe the elastic and inelastic collision processes. We find that the loss behavior exhibits a n3{n}^{3}n3 and an anomalous n2{n}^{2}n2 density dependence for a ratio of elastic-to-inelastic collision rate larger and smaller than 1, respectively. The corresponding evolutions of the energy distribution show collisional cooling or evolution toward low-energetic nonthermalized steady states, respectively. These findings are particularly relevant for understanding atom loss and the energetic evolution of ultracold gases of fermionic lithium atoms in their ground state.
We experimentally investigate the unitarity-limited behavior of the three-body loss near a ppp-wave Feshbach resonance in a single-component Fermi gas of 6Li^{6}\mathrm{Li}6Li atoms. At the unitarity limit, the three-body loss coefficient L3{L}_{3}L3​ exhibits universality in the sense that it is independent of the interaction strength and follows the predicted temperature scaling law of ${L}_{3}\ensuremath{\propto}{T}^{\ensuremath{-}2}$. When decreasing the interaction strength from the unitarity regime, the three-body loss coefficient as a function of the interaction strength and temperature can be described by the theory based on the association of an excited resonant quasibound state and its relaxation into a deep stable dimer by collision with a third atom in the framework of the standard Breit-Wigner theoretical approach. The results reported here are important to understand the properties of a resonant ppp-wave Fermi gas in the prospect of quantum few- and many-body physics.
We study collisional loss of a quasi-one-dimensional spin-polarized Fermi gas near a p-wave Feshbach resonance in ultracold ^{6}Li atoms. We measure the location of the p-wave resonance in quasi-1D and observe a confinement-induced shift and broadening. We find that the three-body loss coefficient L_{3} as a function of the quasi-1D confinement has little dependence on confinement strength. We also analyze the atom loss with a two-step cascade three-body loss model in which weakly bound dimers are formed prior to their loss arising from atom-dimer collisions. Our data are consistent with this model. We also find a possible suppression in the rate of dimer relaxation with strong quasi-1D confinement. We discuss the implications of these measurements for observing p-wave pairing in quasi-1D.
Achieving precise control of ultracold atomic gases requires a detailed understanding of atom loss mechanisms. Motivated by the anomalous three-body decay in a three-component Fermi gas reported in Ref. [1], this work investigates mechanisms that possibly contribute to the observed loss. The three-body Schrödinger equation is solved in the hyperspherical adiabatic representation with pairwise van der Waals interactions, and the SSS-matrix is obtained via the eigenchannel RRR-matrix method to compute recombination rate coefficients K3K_3K3​ and two-body cross sections. At the magnetic field strength where the anomalous decay occurs, K3K_3K3​ is unitary limited, exhibiting the threshold energy scaling K3(E)∝E−1K_3(E)\propto E^{-1}K3​(E)∝E−1. Consequently, the thermally averaged ⟨K3⟩\langle K_3 \rangle⟨K3​⟩ acquires a temperature dependence. Because the experiment is performed in the degenerate regime, ⟨K3⟩\langle K_3 \rangle⟨K3​⟩ also explicitly depends on the per-spin densities through the per-spin Fermi energies EF(i)∝ni2/3E_{F}^{(i)}\propto n_i^{2/3}EF(i)​∝ni2/3​. As the gas is diluted and degeneracy is reduced, ⟨K3⟩\langle K_3 \rangle⟨K3​⟩ approaches the non-degenerate value and becomes a function of temperature only. Channel-resolved branching ratios and cross sections are folded into a Monte Carlo cascade simulation of secondary collisions and trap escape. The analysis indicates that typical three-body recombination events remove fewer than three atoms on average, and that the atom losses are primarily due to the ejection of secondary collision products, rather than the initial three-body recombination products. Therefore, a significant fraction of the released binding energy remains in the trapped ensemble as kinetic energy. Retained energy drives evaporative loss, offering a plausible, partial explanation for the anomalous decay.
We study Efimov resonances via three-body loss in an ultracold two-component gas of fermionic 6Li^{6}\mathrm{Li}6Li and bosonic 133Cs^{133}\mathrm{Cs}133Cs atoms close to a Feshbach resonance at 843 G, extending results reported previously [Pires et al., Phys. Rev. Lett. 112, 250404 (2014)] to temperatures around 120 nK. The experimental scheme for reaching lower temperatures is based upon compensating the gravity-induced spatial separation of the mass-imbalanced gases with bichromatic optical dipole traps. We observe the first and second excited Li-Cs-Cs Efimov resonance in the magnetic field dependence of the three-body event rate constant, in good agreement with the universal zero-range theory at finite temperature [Petrov and Werner, Phys. Rev. A 92, 022704 (2015)]. Deviations are found for the Efimov ground state, and the inelasticity parameter $\ensuremath{\eta}$ is found to be significantly larger than those for single-species systems.
Two-body inelastic collisions arising from chemical reactions are prevalent in ultracold fermionic and bosonic molecular gases. Although recent advancements have achieved quantum degeneracy in these systems, loss dynamics are typically modeled phenomenologically using rate equations that often assume thermalization during chemical reactions. In this study, we employ the inelastic quantum Boltzmann equation to analyze particle loss, temperature evolution, and momentum distributions in single-component Fermi gases from first principles. Our results demonstrate that the conventional particle-number rate equation accurately describes the dynamics in trapped systems but fails to capture the behavior in homogeneous systems. Notably, under pure p-wave inelastic collisions and zero elastic collisions, we find that systems prepared near or above quantum degeneracy remain in a thermal state, whereas systems initialized deep within degeneracy exhibit nonequilibrium dynamics. Our theoretical predictions align well with recent experimental observations in trapped systems, and our claim can be further verified in atomic systems with induced two-body loss in box potentials.