arXiv subjects

Joachim Siemund

Publications and source records attributed to Joachim Siemund.

4 recordsLinked to original sources

arXiv2026

Sub-barrier peaks in atom-atom-ion three-body recombination

Hybrid atom-ion experiments have recently entered the few-partial wave regime in which magnetically tunable Feshbach resonances dominate the three-body loss dynamics. This necessitates a treatment of three-body recombination that can incorporate the role of the resonant dimer channel. Here, we develop a two-step Lindemann-type model for resonant atom-atom-ion three-body recombination that combines multichannel quantum-defect theory for the long-range polarization potential with a universal quantum-capture treatment of the inelastic atom-dimer step. We show that within our model, rate competition can lead to a peak in the three-body-recombination cross section of higher partial-wave channels at energies well below their corresponding centrifugal barrier. Investigating the resulting loss rates under numerically obtained non-thermal collision-energy distributions, we find that these effects survive the energy averaging and can be observed in state-of-the-art hybrid experimental setups. Our findings pave the way for a detailed characterization of atom-ion Feshbach resonances via the analysis of inelastic three-body-recombination processes.

physics.atom-ph↗

arXiv2026

Quantum statistics on atom-ion Feshbach resonances

We investigate three-body recombination in a hybrid atom-ion system consisting of a single trapped Ba$^+$ ion immersed in a two-component Fermi gas of Li atoms near an atom-ion Feshbach resonance. By tuning the spin composition at constant density and temperature, we isolate the role of quantum statistics in atom-atom-ion collisions. The measured ion loss rate exhibits a pronounced nonlinear dependence on spin polarization, revealing a reduced contribution of recombination pathways involving identical fermions already at the level of experimental observables. The observations are consistent with a two-step recombination picture and an adiabatic hyperspherical approach, where antisymmetrization restricts the available entrance channels and gives rise to interference between indistinguishable recombination pathways. Our work establishes atom-ion systems as a platform for controlling three-body collisions via quantum statistics and demonstrates that exchange-symmetry effects remain robust even under thermal averaging that obscures the underlying threshold-law behavior.

physics.atom-ph↗

arXiv2025

Magnetic Feshbach resonances in Ba$^+$+Li collisions due to strong spin-orbit coupling

We report a pronounced dependence of magnetic Feshbach resonance spectra on the initial hyperfine-Zeeman state of Li in ultracold $^{138}$Ba$^+$+$^6$Li collisions. The measured number and distribution of resonances differ significantly between the two lowest states despite their similar electron spin character. We address this puzzle by developing a comprehensive yet generic computational model calibrated against key statistical features in the experimental spectrum. We confirm that strong spin-orbit coupling induces essential changes in the distribution of resonances, leading to an increase in the number of resolvable resonances. Our model reproduces the statistics of the spectrum with the lowest Li state but struggles with the second-lowest state, where a significantly smaller number of resonances is experimentally observed.

physics.atom-ph↗

arXiv2025

Exploring atom-ion Feshbach resonances below the s-wave limit

Revealing the quantum properties of matter requires a high degree of experimental control accompanied by a profound theoretical understanding. At ultracold temperatures, quantities that appear continuous in everyday life, such as the motional angular momentum of two colliding particles, become quantized, leaving a measurable imprint on experimental results. Embedding a single particle within a larger quantum bath at lowest temperatures can result in resonant partial-wave dependent interaction, whose strength near zero energy is dictated by universal threshold scaling laws. Hybrid atom-ion systems have emerged as a novel platform in which a single charged atom in an ultracold bath serves as a well-controlled impurity of variable energy. However, entering the low-energy s-wave regime and exploring the role of higher-partial-wave scattering within has remained an open challenge. Here, we immerse a Barium ion in a cloud of ultracold spin-polarized Lithium atoms, realize tunable collision energies below the s-wave limit and explore resonant higher-partial-wave scattering by studying the energy dependence of Feshbach resonances. Utilizing precise electric field control, we tune the collision energy over four orders of magnitude, reaching from the many-parital-wave to the s-wave regime. At the lowest energies, we probe the energy dependence of an isolated s-wave Feshbach resonance and introduce a theoretical model that allows to distinguish it from higher-partial-wave resonances. Additionally, at energies around the p-wave barrier, we find and identify an open-channel f-wave resonance, consistent with threshold laws. Our findings highlight and benchmark the importance of higher-partial-wave scattering well within the s-wave regime and offer control over chemical reactions and complex many-body dynamics in atom-ion ensembles - on the level of individual angular momentum quanta.

physics.atom-ph↗