Feshbach loss spectroscopy in an ultracold ${}^{23}\mathrm{Na}-{}^{40}\mathrm{K}$ mixture

We perform Feshbach spectroscopy in an ultracold mixture of ${}^{23}\mathrm{Na}$ and ${}^{40}\mathrm{K}$ with different spin-state combinations. We observed 24 new interspecies Feshbach resonances at magnetic fields up to 350 G. A full coupled-channel calculation is performed to assign these resonances. Among them, 12 resonances are identified as $d$-wave Feshbach resonances. These $d$-wave Feshbach resonances are about 5 G systematically smaller than the predictions based on the previous model potential. Taking into account these experimental results, we improve the Born-Oppenheimer potentials between Na and K and achieve good agreement between the theory and experiment for all the observed Feshbach resonances.

Figure 1

Illustration of the experimental setup. The Zeeman slower and two-dimensional magneto-optical traps produce a slow flux of ${}^{23}\mathrm{Na}$ and ${}^{40}\mathrm{K}$ atoms, respectively. The atoms are captured in the main chamber by a two-species dark MOT and subsequently transferred into a cloverleaf magnetic trap for evaporative cooling. In the final stage, the atoms are loaded into a crossed-beam dipole trap for further evaporative cooling to achieve an ultracold Bose-Fermi mixture.

Figure 2

(a) and (b) Absorption time-of-flight (TOF) images of the quantum degenerate Bose-Fermi mixture. (c) and (d) Center-sliced column density of the bosonic and fermionic clouds. The solid lines in (c) and (d) are bimodel and Fermi-Dirac fitting curves for Na and K, respectively.

Figure 3

Observed loss spectrum as a function of the magnetic field. The solid lines are the Gaussian fitting curves. (a) Broad $d$-wave Feshbach resonance near 283 G between Na in the $|1,1\rangle$ state and K in the $|9/2,3/2\rangle$ state accompanied by a slightly narrower $s$-wave resonance. (b) Similar to (a) but for the pair of Na in the $|1,1\rangle$ state and K in the $|9/2,1/2\rangle$ state. These $d$-wave resonances show up by enhanced two-body inelastic collisions (see the calculated collision rate constants in Fig. 4). Also shown are narrow $d$-wave Feshbach resonances near 200 G between Na in the $|1,1\rangle$ state and K in the (c) $|9/2,-9/2\rangle$ and (d) $|9/2,-1/2\rangle$ states. These resonances are observed through three-body losses.

Figure 4

Calculated Feshbach resonances for two different kinetic energies given in $\mu \mathrm{K}$ between Na in the $|1,1\rangle$ state and K in the $|9/2,1/2\rangle$ state. The feature at 279.5 G results in the observed $d$-wave resonance and that at 327.7 G is the $s$-wave resonance. The elastic and inelastic channels are labeled by atom-pair quantum numbers in the legend.