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Enhanced magnetoassociation of $^6$Li in the quantum degenerate regime

Vineetha Naniyil, Yijia Zhou, Guy Simmonds, N. Cooper, Weibin Li, Lucia Hackermüller

Published Feb 3, 2021• 0 citations
OpenAlex

Abstract

We study magnetic Feshbach resonance of ultracold $^6$Li atoms in a dipole trap close to quantum degeneracy. The experiment is carried out by linearly ramping down the magnetic field from the BCS to the BEC side around the broad resonance at $B_r=834.1$G. The Feshbach molecule formation efficiency depends strongly on the temperature of the atomic gas and the rate at which the magnetic field is ramped across the Feshbach resonance. The molecular association process is well described by the Landau-Zener transition while above the Fermi temperature, such that two-body physics dominates the dynamics. However, we observe an enhancement of the atom-molecule coupling as the Fermionic atoms reach degeneracy, demonstrating the importance of many-body coherence not captured by the conventional Landau-Zener model. We develop a theoretical model that explains the temperature dependence of the atom-molecule coupling. Furthermore, we characterize this dependence experimentally and extract the atom-molecule coupling coefficient as a function of temperature, finding qualitative agreement between our model and experimental results. Accurate measurement of this coupling coefficient is important for both theoretical and experimental studies of atom-molecule association dynamics.

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Abstract The association process of Feshbach molecules is well described by a Landau–Zener (LZ) transition above the Fermi temperature, such that two-body physics dominates the dynamics. However, using 6Li atoms and the associated Feshbach resonance at B r = 834.1 G, we observe an enhancement of the atom–molecule coupling as the fermionic atoms reach degeneracy, demonstrating the importance of many-body coherence not captured by the conventional LZ model. In the experiment, we apply a linear association ramp ranging from adiabatic to non-equilibrium molecule association for various temperatures. We develop a theoretical model that explains the temperature dependence of the atom–molecule coupling. Furthermore, we characterize this dependence experimentally and extract the atom–molecule coupling coefficient as a function of temperature, finding qualitative agreement between our model and experimental results. In addition, we simulate the dynamics of molecular association during a nonlinear field ramp. We find that, in the non-equilibrium regime, molecular association efficiency can be enhanced by sweeping the magnetic field cubically with time. Accurate measurement of the atom–molecule coupling coefficient is important for both theoretical and experimental studies of molecular association and many-body collective dynamics.

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Adiabatic association of ultracold molecules via magnetic-field tunable interactions

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We consider in detail the situation of applying a time-dependent external magnetic field to a 87 Rb atomic Bose–Einstein condensate held in a harmonic trap, in order to adiabatically sweep the interatomic interactions across a Feshbach resonance to produce diatomic molecules. To this end, we introduce a minimal two-body Hamiltonian depending on just five measurable parameters of a Feshbach resonance, which accurately determines all low-energy binary scattering observables, in particular, the molecular conversion efficiency of just two atoms. Based on this description of the microscopic collision phenomena, we use the many-body theory of Köhler and Burnett (2002 Phys. Rev. A 65 033601) to study the efficiency of the association of molecules in a 87 Rb Bose–Einstein condensate during a linear passage of the magnetic-field strength across the 100 mT Feshbach resonance. We explore different, experimentally accessible, parameter regimes, and compare the predictions of Landau–Zener, configuration interaction, and two-level mean-field calculations with those of the microscopic many-body approach. Our comparative studies reveal a remarkable insensitivity of the molecular conversion efficiency with respect to both the details of the microscopic binary collision physics and the coherent nature of the Bose–Einstein condensed gas, provided that the magnetic-field strength is varied linearly. We provide the reasons for this universality of the molecular production achieved by linear ramps of the magnetic-field strength, and identify the Landau–Zener coefficient determined by Mies et al (2000 Phys. Rev. A 61 022721) as the main parameter that controls the efficiency.

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We create weakly bound Li/sub 2/ molecules from a degenerate two component Fermi gas by sweeping a magnetic field across a Feshbach resonance. The atom-molecule transfer efficiency can reach 85% and is studied as a function of magnetic field and initial temperature. The bosonic molecules remain trapped for 0.5 s and their temperature is within a factor of 2 from the Bose-Einstein condensation temperature. A thermodynamical model reproduces qualitatively the experimental findings

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An interacting Fermi-Fermi mixture at the crossover of a narrow Feshbach resonance

L. Costa

This work describes experiments with quantum-degenerate atomic mixtures at ultracold temperatures, where quantum statistics determine macroscopic system properties. The first heteronuclear molecules at ultracold temperatures are formed in a quantum degenerate two-species Fermi-Fermi mixture on the repulsive side of a narrow s-wave Feshbach resonance. Elastic collisions in this mixture are investigated with the method of cross-dimensional relaxation. Long-lived two-body bound states on the atomic side of the resonance are detected due to a many-body effect at the crossover of the narrow Feshbach resonance. In addition, atom scattering with fermionic 40K on a light field grating in the Bragg and Kapitza-Dirac regimes is realized for the first time. The versatile experimental platform, where the investigations are done, offers the possibility to perform studies on mixtures involving the bosonic species 87Rb and the two fermionic species 6Li and 40K. Within this work, mainly interactions between the two fermionic species are considered. A quantum-degenerate mixture of 6Li and 40K can be used to create heteronuclear bosonic molecules close to an interspecies s-wave Feshbach resonance. By an adiabatic magnetic field sweep, up to 4 × 10^4 molecules are produced with conversion efficiencies close to 50%. A direct and sensitive molecule detection method is developed to probe molecule properties. The lifetime of the molecules in an atom-molecule mixture exhibits a strong magnetic field dependence. Close to resonance, lifetimes of more than 100ms are observed what offers excellent starting conditions for further investigation and manipulation of the molecular cloud. The interspecies Feshbach resonance, which serves for the production of molecules, is further characterized. The method of cross-dimensional relaxation is applied for the first time to a Fermi-Fermi mixture. For this method, a non-equilibrium state is created, which rethermalizes by pure interspecies collisions due to the fermionic nature of the two species. The lighter atomic species, 6Li, relaxes faster in the mixture than the heavier one, 40K. This is verified by an analytical model, Monte-Carlo simulations, and measurements. With this technique, elastic scattering cross sections are measured over a wide range of magnetic field strengths across the Feshbach resonance. The position (B0 = 154.71(5)G) and the magnetic field width of the Feshbach resonance (Delta = 1.02(7)G) are determined. By comparison of the several measurements, long-lived bound states exist on the atomic side of the resonance due to a many-body effect in the crossover regime of the resonance. In addition, atomic scattering with ultracold 40K on a light field crystal is studied for the first time. The light grating is generated by two counter-propagating laser beams. Suitable pulse parameters for the realization of atom scattering in the Bragg and Kapitza-Dirac regime are found. The momentum spread of the cloud determines the efficiency of the scattering process, which is increased by lowering the temperature of the system.

•Jan 1, 2011•1 citation

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The problem of molecular production from a degenerate gas of fermions at a broad Feshbach resonance, in a single-mode approximation, is reduced to the linear Landau-Zener problem for operators. The strong interaction and induced coherence lead to significant renormalization of the gap between adiabatic levels. The essential region of energies near the resonance contributing to the atom-molecule transformation strongly exceeds the Fermi energy of the weakly interacting gas. The close vicinity of the exact resonance does not play as substantial a role as in the static problem. Our main results are: i) The molecular production is sensitive to the initial magnetic field. ii) In the inverse process of molecule dissociation a large BCS condensate distributed over a broad range of momenta is generated.

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Conversion of a Degenerate Fermi Gas of 6Li Atoms to a Molecular BEC

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Atomic Feshbach resonances have recently been used to produce a strongly interacting Fermi gas, where the BCS/BEC crossover can be explored. We have used both narrow and broad Feshbach resonances to convert a quantum degenerate Fermi gas of 6Li atoms into an ultracold gas of Li2 molecules. For the narrow resonances, the molecules are formed by coherent adiabatic passage through the resonance. We find that 50% of the atoms are converted to molecules. Furthermore, the lifetime of these molecules was measured to be surprisingly long, 1 s. We will discuss these measurements in the context of the present theoretical understanding. Molecules can also be formed using static fields near the broad Feshbach resonance. The lifetime of these molecules is again long, and sufficient to enable their evaporation to a Bose‐Einstein condensate. Phase contrast images of the molecular condensate are presented. The BCS/BEC crossover may be explored by starting with a pure molecular condensate on the low‐field side of the Feshbach resonance, and adiabatically changing the field to any final value around resonance. We combine this ability with optical spectroscopy on a bound‐bound molecular transition to probe the nature of the many‐body wavefunction in the crossover regime.

AIP conference proceedings•Jan 1, 2005•0 citations
Enhanced magnetoassociation of 6^66Li in the quantum degenerate regime
We study magnetic Feshbach resonance of ultracold 6^66Li atoms in a dipole trap close to quantum degeneracy. The experiment is carried out by linearly ramping down the magnetic field from the BCS to the BEC side around the broad resonance at Br=834.1B_r=834.1Br​=834.1G. The Feshbach molecule formation efficiency depends strongly on the temperature of the atomic gas and the rate at which the magnetic field is ramped across the Feshbach resonance. The molecular association process is well described by the Landau-Zener transition while above the Fermi temperature, such that two-body physics dominates the dynamics. However, we observe an enhancement of the atom-molecule coupling as the Fermionic atoms reach degeneracy, demonstrating the importance of many-body coherence not captured by the conventional Landau-Zener model. We develop a theoretical model that explains the temperature dependence of the atom-molecule coupling. Furthermore, we characterize this dependence experimentally and extract the atom-molecule coupling coefficient as a function of temperature, finding qualitative agreement between our model and experimental results. Accurate measurement of this coupling coefficient is important for both theoretical and experimental studies of atom-molecule association dynamics.
Abstract The association process of Feshbach molecules is well described by a Landau–Zener (LZ) transition above the Fermi temperature, such that two-body physics dominates the dynamics. However, using 6Li atoms and the associated Feshbach resonance at B r = 834.1 G, we observe an enhancement of the atom–molecule coupling as the fermionic atoms reach degeneracy, demonstrating the importance of many-body coherence not captured by the conventional LZ model. In the experiment, we apply a linear association ramp ranging from adiabatic to non-equilibrium molecule association for various temperatures. We develop a theoretical model that explains the temperature dependence of the atom–molecule coupling. Furthermore, we characterize this dependence experimentally and extract the atom–molecule coupling coefficient as a function of temperature, finding qualitative agreement between our model and experimental results. In addition, we simulate the dynamics of molecular association during a nonlinear field ramp. We find that, in the non-equilibrium regime, molecular association efficiency can be enhanced by sweeping the magnetic field cubically with time. Accurate measurement of the atom–molecule coupling coefficient is important for both theoretical and experimental studies of molecular association and many-body collective dynamics.
We consider in detail the situation of applying a time-dependent external magnetic field to a 87 Rb atomic Bose–Einstein condensate held in a harmonic trap, in order to adiabatically sweep the interatomic interactions across a Feshbach resonance to produce diatomic molecules. To this end, we introduce a minimal two-body Hamiltonian depending on just five measurable parameters of a Feshbach resonance, which accurately determines all low-energy binary scattering observables, in particular, the molecular conversion efficiency of just two atoms. Based on this description of the microscopic collision phenomena, we use the many-body theory of Köhler and Burnett (2002 Phys. Rev. A 65 033601) to study the efficiency of the association of molecules in a 87 Rb Bose–Einstein condensate during a linear passage of the magnetic-field strength across the 100 mT Feshbach resonance. We explore different, experimentally accessible, parameter regimes, and compare the predictions of Landau–Zener, configuration interaction, and two-level mean-field calculations with those of the microscopic many-body approach. Our comparative studies reveal a remarkable insensitivity of the molecular conversion efficiency with respect to both the details of the microscopic binary collision physics and the coherent nature of the Bose–Einstein condensed gas, provided that the magnetic-field strength is varied linearly. We provide the reasons for this universality of the molecular production achieved by linear ramps of the magnetic-field strength, and identify the Landau–Zener coefficient determined by Mies et al (2000 Phys. Rev. A 61 022721) as the main parameter that controls the efficiency.
This work describes experiments with quantum-degenerate atomic mixtures at ultracold temperatures, where quantum statistics determine macroscopic system properties. The first heteronuclear molecules at ultracold temperatures are formed in a quantum degenerate two-species Fermi-Fermi mixture on the repulsive side of a narrow s-wave Feshbach resonance. Elastic collisions in this mixture are investigated with the method of cross-dimensional relaxation. Long-lived two-body bound states on the atomic side of the resonance are detected due to a many-body effect at the crossover of the narrow Feshbach resonance. In addition, atom scattering with fermionic 40K on a light field grating in the Bragg and Kapitza-Dirac regimes is realized for the first time. The versatile experimental platform, where the investigations are done, offers the possibility to perform studies on mixtures involving the bosonic species 87Rb and the two fermionic species 6Li and 40K. Within this work, mainly interactions between the two fermionic species are considered. A quantum-degenerate mixture of 6Li and 40K can be used to create heteronuclear bosonic molecules close to an interspecies s-wave Feshbach resonance. By an adiabatic magnetic field sweep, up to 4 × 10^4 molecules are produced with conversion efficiencies close to 50%. A direct and sensitive molecule detection method is developed to probe molecule properties. The lifetime of the molecules in an atom-molecule mixture exhibits a strong magnetic field dependence. Close to resonance, lifetimes of more than 100ms are observed what offers excellent starting conditions for further investigation and manipulation of the molecular cloud. The interspecies Feshbach resonance, which serves for the production of molecules, is further characterized. The method of cross-dimensional relaxation is applied for the first time to a Fermi-Fermi mixture. For this method, a non-equilibrium state is created, which rethermalizes by pure interspecies collisions due to the fermionic nature of the two species. The lighter atomic species, 6Li, relaxes faster in the mixture than the heavier one, 40K. This is verified by an analytical model, Monte-Carlo simulations, and measurements. With this technique, elastic scattering cross sections are measured over a wide range of magnetic field strengths across the Feshbach resonance. The position (B0 = 154.71(5)G) and the magnetic field width of the Feshbach resonance (Delta = 1.02(7)G) are determined. By comparison of the several measurements, long-lived bound states exist on the atomic side of the resonance due to a many-body effect in the crossover regime of the resonance. In addition, atomic scattering with ultracold 40K on a light field crystal is studied for the first time. The light grating is generated by two counter-propagating laser beams. Suitable pulse parameters for the realization of atom scattering in the Bragg and Kapitza-Dirac regime are found. The momentum spread of the cloud determines the efficiency of the scattering process, which is increased by lowering the temperature of the system.
The problem of molecular production from a degenerate gas of fermions at a broad Feshbach resonance, in a single-mode approximation, is reduced to the linear Landau-Zener problem for operators. The strong interaction and induced coherence lead to significant renormalization of the gap between adiabatic levels. The essential region of energies near the resonance contributing to the atom-molecule transformation strongly exceeds the Fermi energy of the weakly interacting gas. The close vicinity of the exact resonance does not play as substantial a role as in the static problem. Our main results are: i) The molecular production is sensitive to the initial magnetic field. ii) In the inverse process of molecule dissociation a large BCS condensate distributed over a broad range of momenta is generated.
Atomic Feshbach resonances have recently been used to produce a strongly interacting Fermi gas, where the BCS/BEC crossover can be explored. We have used both narrow and broad Feshbach resonances to convert a quantum degenerate Fermi gas of 6Li atoms into an ultracold gas of Li2 molecules. For the narrow resonances, the molecules are formed by coherent adiabatic passage through the resonance. We find that 50% of the atoms are converted to molecules. Furthermore, the lifetime of these molecules was measured to be surprisingly long, 1 s. We will discuss these measurements in the context of the present theoretical understanding. Molecules can also be formed using static fields near the broad Feshbach resonance. The lifetime of these molecules is again long, and sufficient to enable their evaporation to a Bose‐Einstein condensate. Phase contrast images of the molecular condensate are presented. The BCS/BEC crossover may be explored by starting with a pure molecular condensate on the low‐field side of the Feshbach resonance, and adiabatically changing the field to any final value around resonance. We combine this ability with optical spectroscopy on a bound‐bound molecular transition to probe the nature of the many‐body wavefunction in the crossover regime.