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arXiv:physics/0207098 · Physical Review Letters

Magnetic Field Control of Elastic Scattering in a Cold Gas of Fermionic Lithium Atoms

Selim Jochim, M. Bartenstein, G. Hendl, Johannes Hecker Denschlag, Rudolf Grimm, Allard P. Mosk, Matthias Weidemüller

Published Dec 18, 2002• 83 citations
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Abstract

We study elastic collisions in an optically trapped spin mixture of fermionic lithium atoms in the presence of magnetic fields up to 1.5 kG by measuring evaporative loss. Our experiments confirm the expected magnetic tunability of the scattering length by showing the main features of elastic scattering according to recent calculations. We measure the zero crossing of the scattering length at 530(3) G which is associated with a predicted Feshbach resonance at approximately 850 G. Beyond the resonance we observe the expected large cross section in the triplet scattering regime.

DOI
10.1103/physrevlett.89.273202

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We study elastic collisions in an optically trapped spin mixture of fermionic lithium atoms in the presence of magnetic fields up to 1.5 kG by measuring evaporative loss. Our experiments confirm the expected magnetic tunability of the scattering length by showing the main features of elastic scattering according to recent calculations. We measure the zero crossing of the scattering length at 530(3) G which is associated with a predicted Feshbach resonance at approximately 850 G. Beyond the resonance we observe the expected large cross section in the triplet scattering regime.
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We use an all-optical trap to confine a strongly attractive two-state mixture of lithium fermions. By measuring the rate of evaporation from the trap, we determine the effective elastic scattering cross section 4pia(2) to show that the magnitude of the scattering length |a| is very large, in agreement with predictions. We show that the mixture is stable against inelastic decay provided that a small bias magnetic field is applied. For this system, the s-wave interaction is widely tunable at low magnetic field, and can be turned on and off rapidly via a Raman pi pulse. Hence, this mixture is well suited for fundamental studies of an interacting Fermi gas.
Three magnetic-field induced heteronuclear Feshbach resonances were identified in collisions between bosonic 87Rb^{87}\mathrm{R}\mathrm{b}87Rb and fermionic 40K^{40}\mathrm{K}40K atoms in their absolute ground states. Strong inelastic loss from an optically trapped mixture was observed at the resonance positions of 492, 512, and $543\ifmmode\pm\else\textpm\fi{}2\text{ }\text{ }\mathrm{G}$. The magnetic-field locations of these resonances place a tight constraint on the triplet and singlet cross-species scattering lengths, yielding $(\ensuremath{-}281\ifmmode\pm\else\textpm\fi{}15){a}_{0}$ and $(\ensuremath{-}54\ifmmode\pm\else\textpm\fi{}12){a}_{0}$, respectively. The width of the loss feature at 543 G is $3.7\ifmmode\pm\else\textpm\fi{}1.5\text{ }\text{ }\mathrm{G}$ wide; this broad Feshbach resonance should enable experimental control of the interspecies interactions.
We have loaded an ultracold gas of fermionic atoms into a far-off resonance optical dipole trap and precisely controlled the spin composition of the trapped gas. We have measured a magnetic-field Feshbach resonance between atoms in the two lowest energy spin states, /9/2,-9/2> and /9/2,-7/2>. The resonance peaks at a magnetic field of 201.5+/-1.4 G and has a width of 8.0+/-1.1 G. Using this resonance, we have changed the elastic collision cross section in the gas by nearly 3 orders of magnitude.
We measure a zero crossing in the scattering length of a mixture of the two lowest hyperfine states of 6Li.{}^{6}\mathrm{Li}.6Li. To locate the zero crossing, we monitor the decrease in temperature and atom number arising from evaporation in a CO2{\mathrm{CO}}_{2}CO2​ laser trap as a function of magnetic field B. The temperature decrease and atom loss are minimized for $B=52.8\ifmmode\pm\else\textpm\fi{}0.4\mathrm{mT},$ consistent with no evaporation. We also present preliminary calculations using potentials that have been constrained by the measured zero crossing and locate a broad Feshbach resonance at $\ensuremath{\approx}86\mathrm{mT},$ in agreement with previous theoretical predictions. In addition, our theoretical model predicts a second and much narrower Feshbach resonance near 55 mT.
We studied the magnetic field dependence of the inelastic decay of an ultracold, optically trapped fermionic 6Li gas of different spin compositions. The spin mixture of the two lowest hyperfine states showed two decay resonances at 550 and 680 G, consistent with the predicted Feshbach resonances for elastic s-wave collisions. The observed lifetimes of several hundred ms are much longer than the expected time for Cooper pair formation and the phase transition to superfluidity in the vicinity of the Feshbach resonance.