Broad Feshbach resonance with a large background scattering length in a fermionic atom-molecule mixture

TL;DR

The work reports a broad Feshbach resonance at around $B_0\simeq 217.3$ G in a mass-imbalanced fermionic mixture of $^{23}$Na$^{40}$K molecules and $^{40}$K atoms in their absolute ground states, featuring an unusually large negative background scattering length $a_{\rm bg}\approx -2902\,a_0$ and width $\Delta\simeq -23.1$ G, yielding a strong open-channel character with $A=a_{\rm bg}\Delta/a_0\approx 6.6\times 10^4$ G. Elastic cross sections, inferred from cross-species thermalization, reveal strong interactions even far from resonance, driving the system into a hydrodynamic regime where atom and molecule center-of-mass motions lock to a common frequency around $\widetilde{\omega}\approx 2\pi\times 254$ Hz. This resonance enables exploration of strongly interacting mass-imbalanced fermionic gases and potential BEC-BCS crossover physics, while addressing challenges such as molecule lifetimes near resonance and proposing strategies to suppress photoexcitation losses using longer-wavelength trapping light.

Abstract

We report the observation of a broad magnetic Feshbach resonance with a large background scattering length in an ultracold fermionic mixture of $^{23}$Na$^{40}$K molecules and $^{40}$K atoms, with both species prepared in their lowest hyperfine states. The Feshbach resonance is characterized by measuring resonantly enhanced loss rates and elastic scattering cross sections via cross-species thermalization. The large background scattering length can drive the atom-molecule mixture into the hydrodynamic regime when the magnetic field is far from the resonance. We observe that the center-of-mass motions of the atoms and molecules are phase-locked and oscillate with a common frequency due to hydrodynamic drag effects. This broad atom-molecule Feshbach resonance with its large background scattering length opens up a new avenue towards studying strongly interacting fermionic gases with mass imbalance.

Figures (6)

• Figure 1: The location of the broad atom-molecule Feshbach resonance. The number of remaining $^{23}$Na$^{40}$K molecules is shown as a function of the magnetic field. The black solid line is a Gaussian fit to the data. The inset shows the loss rate coefficients in the vicinity of the broad Feshbach resonance. The red solid line is a Lorentzian fit to the data. The fit yields the resonance position $B_0=217.3(4)$ G. Error bars shown in the plot are the standard errors.
• Figure 2: Cross-species thermalization measurements at different magnetic fields. The square of the size of the molecular cloud evolves as a function of hold time after heating the atoms by a resonant heating pulse. The red and black points represent the data of $\sigma_x^2$ and $\sigma_y^2$ respectively, and the solid lines represent the fits of the data to the model described in the main text. Error bars shown in the plots are standard errors of the mean.
• Figure 3: The elastic atom-molecule scattering cross sections extracted from the thermalization measurements. The red data points are fitted to the model $\sigma=4\pi|a|^2$ with the solid line. The dashed line is the position of the broad resonance at 217.3 G which is determined from the loss rate coefficients. The fitting parameters are $\Delta=-23.1(2)$ G, $a_{\mathrm{bg}}=-2902(182) a_0$, and $\gamma=2.7(5)$ G. Error bars shown in the plot are the standard errors.
• Figure 4: The center of mass oscillations of atoms and molecules along the gravitational direction in the optical dipole trap. (a) and (b) are the results of pure atomic and molecular clouds, respectively. The two clouds oscillate with their own frequencies in the absence of atom-molecular interaction. (c) and (d) represent the oscillations of atomic and molecular clouds in the atom-molecule mixture. The two clouds oscillate with a common frequency due to the hydrodynamic drag effects.
• Figure S1: Comparison of loss spectrum of pure molecules (blue points) and molecules in the atom-molecule mixture (red points). Error bars represent the standard error of the mean.