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Journal of Physics B Atomic Molecular and Optical Physics

Suppression of interaction-induced loss rate coefficient near broad s-wave and p-wave Feshbach resonances by magnetic field

Bing-Kuan Lyu, Zhi-Xin Sun, Gao‐Ren Wang, Shu‐Lin Cong

Published Feb 24, 2022• 3 citations
Publisher

Abstract

Abstract Ultracold atoms prepared in excited internal states can escape from the trap through inelastic collisions. A magnetic field can be used to suppress the s-wave loss rate coefficient near an s-wave Feshbach resonance. However, at the temperature of μK the p-wave loss rate coefficient becomes increasingly important as the temperature increases. The unsuppressed p-wave loss rate coefficient can far exceed the suppressed s-wave loss rate coefficient, and severely restricts the reduction of the total loss rate coefficient. We calculate the collision rate coefficients of ultracold 85Rb and 87Rb atoms prepared in excited hyperfine and Zeeman states using the coupled-channel method. We find that in R b 85 ( F 1 = 3 , m F 1 = 3 ) + R b 87 ( F 2 = 1 , m F 2 = − 1 ) and R b 85 ( F 1 = 2 , m F 1 = − 2 ) + R b 87 ( F 2 = 2 ,

DOI
10.1088/1361-6455/ac5861

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Abstract Ultracold atoms prepared in excited internal states can escape from the trap through inelastic collisions. A magnetic field can be used to suppress the s-wave loss rate coefficient near an s-wave Feshbach resonance. However, at the temperature of μK the p-wave loss rate coefficient becomes increasingly important as the temperature increases. The unsuppressed p-wave loss rate coefficient can far exceed the suppressed s-wave loss rate coefficient, and severely restricts the reduction of the total loss rate coefficient. We calculate the collision rate coefficients of ultracold 85Rb and 87Rb atoms prepared in excited hyperfine and Zeeman states using the coupled-channel method. We find that in R b 85 ( F 1 = 3 , m F 1 = 3 ) + R b 87 ( F 2 = 1 , m F 2 = − 1 ) and R b 85 ( F 1 = 2 , m F 1 = − 2 ) + R b 87 ( F 2 = 2 ,
Inelastic collision rates for ultracold 85Rb{}^{85}\mathrm{Rb}85Rb atoms in the F=2F\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2F=2, ${m}_{f}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ensuremath{-}2$ state have been measured as a function of magnetic field. At 250 gauss (G), the two- and three-body loss rates were measured to be ${K}_{2}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}(1.87\ifmmode\pm\else\textpm\fi{}0.95\ifmmode\pm\else\textpm\fi{}0.19)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}{\mathrm{cm}}^{3}/\mathrm{s}$ and ${K}_{3}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}({4.24}_{\ensuremath{-}0.29}^{+0.70}\ifmmode\pm\else\textpm\fi{}0.85)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}25}{\mathrm{cm}}^{6}/\mathrm{s}$, respectively. As the magnetic field is decreased from 250 G towards a Feshbach resonance at 155 G, the inelastic rates decrease to a minimum and then increase dramatically, peaking at the Feshbach resonance. Both two- and three-body losses are important, and individual contributions have been compared with theory.
We prepare mixtures of ultracold CaF molecules and Rb atoms in a magnetic trap and study their inelastic collisions. When the atoms are prepared in the spin-stretched state and the molecules in the spin-stretched component of the first rotationally excited state, they collide inelastically with a rate coefficient k_{2}=(6.6±1.5)×10^{-11} cm^{3}/s at temperatures near 100 μK. We attribute this to rotation-changing collisions. When the molecules are in the ground rotational state we see no inelastic loss and set an upper bound on the spin-relaxation rate coefficient of k_{2}<5.8×10^{-12} cm^{3}/s with 95% confidence. We compare these measurements to the results of a single-channel loss model based on quantum defect theory. The comparison suggests a short-range loss parameter close to unity for rotationally excited molecules, but below 0.04 for molecules in the rotational ground state.
Inelastic collision rates for ultracold 85 Rb atoms in the F=2, m f =-2 state have been measured as a function of magnetic field.At 250 Gauss (G), the two-and three-body loss rates were measured to be K 2 =1.87±0.95±0.19x 10 -14 cm 3 /s and K 3 =4.24+0.70 -0.29 ±0.85 x 10 -25 cm 6 /s respectively.As the magnetic field is decreased from 250 G towards a Feshbach resonance at 155 G, the inelastic rates decrease to a minimum and then increase dramatically, peaking at the Feshbach resonance.Both two-and three-body losses are important, and individual contributions have been compared with theory.
This thesis has investigated the dynamics of ultracold atomic collisions in the presence of both static magnetic and oscillating radio-frequency (rf) fields. The bound state structure and scattering length of 39K+133Cs was examined in the presence of only a static magnetic field, where it was found that no Feshbach resonance of suitable width for magnetoassociation existed at a magnetic field where caesium can be cooled to degeneracy. We then showed that zero-energy Feshbach resonances may be engineered using an rf field in places where they did not previously exist. An rf field with frequency 79.9 MHz was chosen to induce a resonance near 21 G. The widths of such rf-induced Feshbach resonances increase quadratically with rf field strength. The resonances presented are lossless with circularly polarized rf, and the molecules created are long-lived even with plane-polarized RF. Collisional losses in rf-dressed magnetic traps were also investigated. An rf-induced loss mechanism that does not exist in the absence of rf radiation was identified. This mechanism is not suppressed by a centrifugal barrier in the outgoing channel, and can be much faster than spin relaxation, which is centrifugally suppressed. We explore the dependence of the rf-induced loss rate on singlet and triplet scattering lengths, hyperfine splittings and the strength of the rf field. The results were interpreted in terms of an adiabatic model of the collision dynamics, and calculate the corresponding nonadiabatic couplings. The loss rate can vary by 10 orders of magnitude as a function of singlet and triplet scattering lengths. 87Rb is a special case, where several factors combine to reduce rf-induced losses; as a result, they are slow compared to spin-relaxation losses. For most other alkali-metal pairs, rf-induced losses are expected to be much faster and may dominate. For heteronuclear mixtures an rf-modified spin-exchange mechanism was identified that results in loss rates orders of magnitude greater than the rf-induced loss rates in homonuclear cases. Fast loss is expected in mixtures where the Lande g-factors differ.
In this work we investigate the potential of controlling cold (O(K)−mK) and ultracold (mK-μK) atom-molecule collisions by tuning scattering states across Feshbach resonances using magnetic fields. We are interested in particular in the prospect of suppressing the often undesirable inelastic collisions. The He-O_2 system provides the vehicle for our study. We calculate bound and quasi-bound states of several isotopic combinations, including their Zeeman structure, to reveal the underlaying pattern for easier characterization of quasi-bound states in terms of rigorous and approximately good quantum numbers. These calculations also help us locate the fields at which zero-energy resonances will occur. Scattering calculations are then performed for collisions of 3^He and 4^He with {16}^O_2 at fixed (1 μK) energy but varying magnetic field. The field is varied to sweep the scattering state across resonance. At low and ultralow energies we enter the Wigner threshold regime where the S-partial wave dominates the wavefunction. The cross sections, and the real and imaginary parts of the scattering length, vary dramatically across resonance. Their profiles are used to analyze the resonances. In a highlight of our results we show that dramatic suppression of inelastic cross sections occur for 4^He-{16}^O_2 . The resonances are relatively wide (of order 100 Gauss), with suppression of inelastic scattering over a similarly wide range of fields and for temperatures ranging from 10 mK down to 1 μK. We conclude that under certain conditions it is possible to almost completely eliminate inelastic collisions. This is potentially very important for cooling techniques, such as evaporative and sympathetic cooling, that require efficient elastic cross sections. Suppression of inelastic collisions can not only increase thermalization efficiency but it can also result in longer trap-lifetimes by reducing transitions to untrapable states.
We have successfully prepared an optically trapped ultracold mixture of 23{}^{23}23Na and 87{}^{87}87Rb atoms and studied their interspecies Feshbach resonances. Using two different spin combinations, several sss- and ppp-wave resonances are identified by observing high inelastic losses and temperature increases for both species near resonant magnetic field values. The two sss-wave resonances observed below 500 G between atoms in their lowest-energy levels are possible candidates for Feshbach molecule association. Our results are well characterized by a coupled-channel model which also refines the ground-state interaction potentials between 23{}^{23}23Na and 87{}^{87}87Rb. This work opens up the prospect for preparing ultracold ensembles of ground-state bosonic NaRb molecules which are chemically stable and can provide strong dipolar interactions.