Abstract
We directly measured the normalized s-wave scattering cross-section of ultracold 40K atoms across a magnetic-field Feshbach resonance by colliding pairs of degenerate Fermi gases (DFGs) and imaging the scattered atoms. We extracted the scattered fraction for a range of bias magnetic fields, and measured the resonance location to be B0 = 20.206(15) mT with width Δ = 1.0(5) mT. To optimize the signal-to-noise ratio (SNR) of atom number in scattering images, we developed techniques to interpret absorption images in a regime where recoil induced detuning corrections are significant. These imaging techniques are generally applicable to experiments with lighter alkalis that would benefit from maximizing SNR on atom number counting at the expense of spatial imaging resolution.
Highlights
Feshbach resonances are widely used for tuning the interaction strength in ultracold atomic gases
In degenerate Fermi gases (DFGs), the tunability of interactions provided by Feshbach resonances has allowed for studies of the creation of molecular Bose-Einstein Condensates (BECs) [1,2,3] as well as observation of the phase transition from the BardeenCooper-Schrieffer (BCS) superconducting regime to the BEC regime at sufficiently low temperatures [4,5,6,7]
For a magnetic-field Feshbach resonance, a bias magnetic field defines the relative energy of the free atomic states in two hyperfine sublevels and the molecular state
Summary
Feshbach resonances are widely used for tuning the interaction strength in ultracold atomic gases. Many experimental techniques have been used to characterize Feshbach resonances These include the observation of atom loss due to threebody inelastic scattering, measurement of re-thermalization timescales, and anisotropic expansion of a cloud upon release from a confining potential, which infer the elastic scattering cross section from collective behavior of the cloud [15,16,17]. We collided pairs of DFGs and imaged the resulting s-wave scattered atoms as a function of bias magnetic field. This allowed us to observe the enhancement in scattering without relying on proxy effects. We measured the fraction of atoms scattered during the collision at different bias magnetic fields and deduced the location and width of the resonance.
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