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arXiv:1209.1418 · Physical Review A

Magnetic Feshbach resonances in collisions of nonmagnetic closed-shell 1Σ molecules

Alisdair O. G. Wallis, Roman V. Krems

Published Mar 31, 2014• 5 citations
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Abstract

Magnetic Feshbach resonances play a central role in experimental research on atomic gases at ultracold temperatures, as they allow one to control the microscopic interactions between ultracold atoms by tuning an applied magnetic field. These resonances arise due to strong hyperfine interactions between the unpaired electron and the nuclear magnetic moment of the alkali metal atoms. A major thrust of current research is to create an ultracold gas of diatomic alkali metal molecules in the ground rovibrational state of the ground electronic ${}^{1}\ensuremath{\Sigma}$ state. Unlike alkali metal atoms, ${}^{1}\ensuremath{\Sigma}$ diatomic molecules have no unpaired electrons. However, the hyperfine interactions of molecules may give rise to magnetic Feshbach resonances. We use quantum scattering calculations to study the possible width of these resonances. Our results show that the widths of magnetic Feshbach resonances in ultracold molecule-molecule collisions for ${}^{1}\ensuremath{\Sigma}$ molecules may exceed 1 mG, rendering such resonances experimentally detectable. We hope that this work will stimulate the experimental search of these resonances.

DOI
10.1103/physreva.89.032716

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Magnetic Feshbach resonances play a central role in experimental research on atomic gases at ultracold temperatures, as they allow one to control the microscopic interactions between ultracold atoms by tuning an applied magnetic field. These resonances arise due to strong hyperfine interactions between the unpaired electron and the nuclear magnetic moment of the alkali metal atoms. A major thrust of current research is to create an ultracold gas of diatomic alkali metal molecules in the ground rovibrational state of the ground electronic 1\ensuremathΣ{}^{1}\ensuremath{\Sigma}1\ensuremathΣ state. Unlike alkali metal atoms, 1\ensuremathΣ{}^{1}\ensuremath{\Sigma}1\ensuremathΣ diatomic molecules have no unpaired electrons. However, the hyperfine interactions of molecules may give rise to magnetic Feshbach resonances. We use quantum scattering calculations to study the possible width of these resonances. Our results show that the widths of magnetic Feshbach resonances in ultracold molecule-molecule collisions for 1\ensuremathΣ{}^{1}\ensuremath{\Sigma}1\ensuremathΣ molecules may exceed 1 mG, rendering such resonances experimentally detectable. We hope that this work will stimulate the experimental search of these resonances.
Magnetic Feshbach resonances in collisions of closed-shell 1Σ^1\Sigma1Σ molecules
Magnetic Feshbach resonances play a central role in experimental research of atomic gases at ultracold temperatures, as they allow one to control the microscopic interactions between ultracold atoms by tuning an applied magnetic field. These resonances arise due to strong hyperfine interactions between the unpaired electron and the nuclear magnetic moment of the alkali metal atoms. A major thrust of current research is to create an ultracold gas of diatomic alkali-metal molecules in the ground rovibrational state of the ground electronic 1Σ^1\Sigma1Σ state. Unlike alkali metal atoms, 1Σ^1\Sigma1Σ diatomic molecules have no unpaired electrons. However, the hyperfine interactions of molecules may give rise to magnetic Feshbach resonances. We use quantum scattering calculations to study the possible width of these resonances. Our results show that the widths of magnetic Feshbach resonances in ultracold molecule-molecule collisions for 1Σ^1\Sigma1Σ molecules may exceed 1 milliGauss, rendering such resonances experimentally detectable. We hope that this work will stimulate the experimental search of these resonances.
We consider the magnetically tunable Feshbach resonances that may exist in ultracold mixtures of molecules in ${}^{2}\mathrm{\ensuremath{\Sigma}}$ states and alkali-metal atoms. We focus on Rb+CaF\mathrm{Rb}+\mathrm{CaF}Rb+CaF as a prototype system. There are likely to be Feshbach resonances analogous to those between pairs of alkali-metal atoms. We investigate the patterns of near-threshold states and the resonances that they cause, using coupled-channel calculations of the bound states and low-energy scattering on model interaction potentials. We explore the dependence of the properties on as-yet-unknown potential parameters. There is a high probability that resonances will exist at magnetic fields below 1000 G, and that these will be broad enough to control collisions and form triatomic molecules by magnetoassociation. We consider the effects of CaF rotation and anisotropy of the interaction potential, and conclude that they may produce additional resonances but should not affect the existence of rotation-free resonances.
We explore the feasibility of producing ultracold diatomic molecules with nonzero electric and magnetic dipole moments by magnetically associating two atoms, one with zero electron spin and one with nonzero spin. Feshbach resonances arise through the dependence of the hyperfine coupling on internuclear distance. We survey the Feshbach resonances in diatomic systems combining the nine stable alkali-metal isotopes with those of Yb, focusing on the illustrative examples of RbYb and CsYb. We show that the resonance widths may expressed as a product of physically comprehensible terms in the framework of Fermi's golden rule. The resonance widths depend strongly on the background scattering length, which may be adjusted by selecting the Yb isotope, and on the hyperfine coupling constant and the magnetic field. In favorable cases the resonances may be over 100 mG wide.
We investigate the interactions between ultracold alkali-metal atoms and closed-shell atoms using electronic structure calculations on the prototype system Rb+Sr. There are molecular bound states that can be tuned across atomic thresholds with a magnetic field and previously neglected terms in the collision Hamiltonian that can produce zero-energy Feshbach resonances with significant widths. The largest effect comes from the interaction-induced variation of the Rb hyperfine coupling. The resonances may be used to form paramagnetic polar molecules if the magnetic field can be controlled precisely enough.
We investigate interactions and ultracold collisions of high-spin but spherical atoms with closed-shell atoms, focusing on the example of Cr with Yb. Such a combination has only one potential energy curve but gives rise to a substantial intra-atomic spin–spin interaction, which causes Feshbach resonances due to rotationally excited states. We find such resonances are guaranteed below 250 G and can reach widths of tens of G in favorable circumstances. We study the effect of hyperfine structure, which can create additional wide resonances at low fields. Finally, we consider isotopic substitution and show that for any realistic potential it is very likely that at least one combination of commonly used isotopes will have resonances with experimentally favorable properties. Thus, this is a promising system for both molecule formation and studying quantum mixtures including dipolar species. Published by the American Physical Society 2024
Magnetically tunable Feshbach resonances for polar paramagnetic ground-state diatomics are too narrow to allow for magnetoassociation starting from trapped, ultracold atoms. We show that nonresonant light can be used to engineer the Feshbach resonances in their position and width. For nonresonant field intensities of the order of 10(9) W/cm(2), we find the width to be increased by 3 orders of magnitude, reaching a few Gauss. This opens the way for producing ultracold molecules with sizable electric and magnetic dipole moments and thus for many-body quantum simulations with such particles.