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Physical Review A

Sympathetic cooling of Li6 atoms

Frank A. van Abeelen, BOUDEWIJN J. VERHAAR, A. J. Moerdijk

Published Jun 1, 1997• 46 citations
Publisher

Abstract

We use recently measured cold photoassociation and two-photon data to extract the singlet and triplet accumulated radial phases of interacting ground-state lithium atoms. Using the resulting values we predict scattering lengths, Feshbach resonances, and exchange decay rates for cold collisions between $^{7}\mathrm{Li}$ and $^{6}\mathrm{Li}$ atoms that are of interest for the possibility of sympathetic cooling of $^{6}\mathrm{Li}$ and for the coexistence of the bosonic and fermionic quantum-degenerate phases of $^{7}\mathrm{Li}$ and $^{6}\mathrm{Li}$. In addition, we calculate scattering lengths and exchange decay rates for cold collisions between identical lithium isotopes in different hyperfine states. These quantities are used to examine the possibilities of coexisting $^{7}\mathrm{Li}$ Bose condensates and of evaporatively cooling coexisting $^{6}\mathrm{Li}$ subsystems.

DOI
10.1103/physreva.55.4377

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This thesis presents studies of quantum degenerate atomic gases of fermionic 6Li and bosonic 7Li. Degeneracy is reached by evaporative cooling of 7Li in a strongly confining magnetic trap. Since at low temperatures direct evaporative cooling is not possible for a polarized fermionic gas, 6Li is sympathetically cooled by thermal contact with 7Li. In a first series of experiments both isotopes are trapped in their low-field seeking higher hyperfine states. A Fermi degeneracy of is achieved for 105 fermions. For more than ~300 atoms, the 7Li condensate collapses, due to the attractive interatomic interaction in this state. This limits the degeneracy reached for both species. To overcome this limit, in a second series of experiments 7Li and 6Li atoms are transferred to their low field seeking lower hyperfine states, where the boson-boson interaction is repulsive but weak. The inter-isotope collisions are used to thermalize the mixture. A 7Li Bose-Einstein condensate (BEC) of 104 atoms immersed in a Fermi sea is produced. The BEC is quasi-one-dimensional and the thermal fraction can be negligible. The measured degeneracies are . The temperature is measured using the bosonic thermal fraction, which vanishes at the lowest temperatures, limiting our measurement sensitivity. In a third series of experiments, the bosons are transferred into an optical trap and their internal state is changed to , the lowest energy state. A Feshbach resonance is detected and used to produce a BEC with tunable atomic interactions. When the effective interaction between atoms is tuned to be small and attractive, we observe the formation of a matter-wave bright soliton. Propagation of the soliton without spreading over a macroscopic distance of 1.1~mm is observed.

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Quantum-Degenerate Mixture of Fermionic Lithium and Bosonic Rubidium Gases

C. Silber, S. Günther, C. Marzok +3 more

We report on the observation of sympathetic cooling of a cloud of fermionic 6Li atoms which are thermally coupled to evaporatively cooled bosonic 87Rb. Using this technique we obtain a mixture of quantum-degenerate gases, where the Rb cloud is colder than the critical temperature for Bose-Einstein condensation and the Li cloud is colder than the Fermi temperature. From measurements of the thermalization velocity we estimate the interspecies s-wave triplet scattering length |amx|=20(+9)(-6)aB. We found that the presence of residual rubidium atoms in the |2, 1> and the |1, -1> Zeeman substates gives rise to important losses due to inelastic collisions.

arXiv:cond-mat/0506217•Physical Review Letters•Oct 20, 2005•133 citations
We use recently measured cold photoassociation and two-photon data to extract the singlet and triplet accumulated radial phases of interacting ground-state lithium atoms. Using the resulting values we predict scattering lengths, Feshbach resonances, and exchange decay rates for cold collisions between 7Li^{7}\mathrm{Li}7Li and 6Li^{6}\mathrm{Li}6Li atoms that are of interest for the possibility of sympathetic cooling of 6Li^{6}\mathrm{Li}6Li and for the coexistence of the bosonic and fermionic quantum-degenerate phases of 7Li^{7}\mathrm{Li}7Li and 6Li^{6}\mathrm{Li}6Li. In addition, we calculate scattering lengths and exchange decay rates for cold collisions between identical lithium isotopes in different hyperfine states. These quantities are used to examine the possibilities of coexisting 7Li^{7}\mathrm{Li}7Li Bose condensates and of evaporatively cooling coexisting 6Li^{6}\mathrm{Li}6Li subsystems.
The electronic structure of the (LiYb)+(\mathrm{LiYb}{)}^{+}(LiYb)+ molecular ion is investigated with two variants of the coupled cluster method restricted to single, double, and noniterative or linear triple excitations. Potential-energy curves for the ground and excited states, permanent and transition electric dipole moments, and long-range interaction coefficients C4{C}_{4}C4​ and C6{C}_{6}C6​ are reported. The data are subsequently employed in scattering calculations and photoassociation studies. Feshbach resonances are shown to be measurable, despite the ion's micromotion in the Paul trap. Molecular ions can be formed in their singlet electronic ground state by one-photon photoassociation and in triplet states by two-photon photoassociation; and control of cold atom-ion chemistry based on Feshbach resonances should be feasible. Conditions for sympathetic cooling of an Yb+{\mathrm{Yb}}^{+}Yb+ ion by an ultracold gas of Li atoms are found to be favorable in the temperature range 10nK−−10mK10\phantom{\rule{0.28em}{0ex}}\mathrm{nK}--10\phantom{\rule{0.28em}{0ex}}\mathrm{mK}10nK−−10mK, and further improvements using Feshbach resonances should be possible. Overall, these results suggest excellent prospects for building a quantum simulator with ultracold Yb+{\mathrm{Yb}}^{+}Yb+ ions and Li atoms.
Photoassociation of ultracold lithium atoms into bound vibrational levels of the $A^{1}\ensuremath{\Sigma}_{u}^{+}$ excited state is used to probe the $X^{1}\ensuremath{\Sigma}_{g}^{+}$ ground-state interaction potential of 6Li2^{6}\mathrm{Li}_{2}6Li2​ and 7Li2^{7}\mathrm{Li}_{2}7Li2​. It had been predicted that the sss-wave photoassociation signal strength would pass through a minimum as a function of vibrational level for positive sss-wave scattering length. We report the observation of this novel effect, and use the location of the minimum to precisely determine the singlet sss-wave scattering length for both isotopes. The sensitivity of this technique is demonstrated by distinguishing the minima for collisions involving 7Li^{7}\mathrm{Li}7Li atoms in different hyperfine states.
Sympathetic cooling of two atomic isotopes is experimentally investigated. Using forced evaporation of a bosonic 7Li{}^{7}\mathrm{Li}7Li gas in a magnetic trap, a sample of $1.3\ifmmode\times\else\texttimes\fi{}{10}^{5}$ 6Li{}^{6}\mathrm{Li}6Li fermions has been sympathetically cooled to $9(3) \ensuremath{\mu}\mathrm{K},$ corresponding to 2.2(0.8)2.2(0.8)2.2(0.8) times the Fermi temperature. The measured rate constant for two-body inelastic collisions of the 7Li{}^{7}\mathrm{Li}7Li ∣2,2〉|2,2〉∣2,2〉 state at low magnetic field is ${1.0}_{\ensuremath{-}0.5}^{+0.8}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14} {\mathrm{cm}}^{3}{\mathrm{s}}^{\ensuremath{-}1}.$
The spectra of the high-lying vibrational levels of the Asp1Sigmasbspu+A\\sp1\\Sigma\\sbsp{u}{+}Asp1Sigmasbspu+ and 1sp3Sigmasbspg+1\\sp3\\Sigma\\sbsp{g}{+}1sp3Sigmasbspg+ singly excited states of both sp6rmLisb2\\sp6\\rm Li\\sb2sp6rmLisb2 and sp7rmLisb2\\sp7\\rm Li\\sb2sp7rmLisb2 are obtained via photoassociation of colliding ultracold atoms confined in a magneto-optical trap. The least bound state of the asp3Sigmasbspu+a\\sp3\\Sigma\\sbsp{u}{+}asp3Sigmasbspu+ ground state potential, obtained by two-photon photoassociative spectroscopy, is also presented for both sp7rmLisb2\\sp7\\rm Li\\sb2sp7rmLisb2 and sp6rmLisb2\\sp6\\rm Li\\sb2sp6rmLisb2. The vibrational levels have resolved hyperfine structure, whose relative energy splittings and transition strengths are accurately modeled. The photoassociative spectra are used to precisely determine both the singlet and triplet s-wave scattering lengths for both isotopes, which are important parameters in systems designed to observe quantum degenerate effects and Bose-Einstein condensation.
This thesis presents studies of quantum degenerate atomic gases of fermionic 6Li and bosonic 7Li. Degeneracy is reached by evaporative cooling of 7Li in a strongly confining magnetic trap. Since at low temperatures direct evaporative cooling is not possible for a polarized fermionic gas, 6Li is sympathetically cooled by thermal contact with 7Li. In a first series of experiments both isotopes are trapped in their low-field seeking higher hyperfine states. A Fermi degeneracy of is achieved for 105 fermions. For more than ~300 atoms, the 7Li condensate collapses, due to the attractive interatomic interaction in this state. This limits the degeneracy reached for both species. To overcome this limit, in a second series of experiments 7Li and 6Li atoms are transferred to their low field seeking lower hyperfine states, where the boson-boson interaction is repulsive but weak. The inter-isotope collisions are used to thermalize the mixture. A 7Li Bose-Einstein condensate (BEC) of 104 atoms immersed in a Fermi sea is produced. The BEC is quasi-one-dimensional and the thermal fraction can be negligible. The measured degeneracies are . The temperature is measured using the bosonic thermal fraction, which vanishes at the lowest temperatures, limiting our measurement sensitivity. In a third series of experiments, the bosons are transferred into an optical trap and their internal state is changed to , the lowest energy state. A Feshbach resonance is detected and used to produce a BEC with tunable atomic interactions. When the effective interaction between atoms is tuned to be small and attractive, we observe the formation of a matter-wave bright soliton. Propagation of the soliton without spreading over a macroscopic distance of 1.1~mm is observed.
We report on the observation of sympathetic cooling of a cloud of fermionic 6Li atoms which are thermally coupled to evaporatively cooled bosonic 87Rb. Using this technique we obtain a mixture of quantum-degenerate gases, where the Rb cloud is colder than the critical temperature for Bose-Einstein condensation and the Li cloud is colder than the Fermi temperature. From measurements of the thermalization velocity we estimate the interspecies s-wave triplet scattering length |amx|=20(+9)(-6)aB. We found that the presence of residual rubidium atoms in the |2, 1> and the |1, -1> Zeeman substates gives rise to important losses due to inelastic collisions.