Quantum degenerate Bose-Fermi mixture of chemically different atomic species with widely tunable interactions

We have created a quantum degenerate Bose-Fermi mixture of ${}^{23}$Na and ${}^{40}$K with widely tunable interactions via broad interspecies Feshbach resonances. Over 30 Feshbach resonances between ${}^{23}$Na and ${}^{40}$K were identified, including $p$-wave multiplet resonances. The large and negative triplet background scattering length between ${}^{23}$Na and ${}^{40}$K causes a sharp enhancement of the fermion density in the presence of a Bose condensate. As explained via the asymptotic bound-state model, this strong background scattering leads to wide Feshbach resonances observed at low magnetic fields. Our work opens up the prospect to create chemically stable, fermionic ground-state molecules of ${}^{23}$Na-${}^{40}$K, where strong, long-range dipolar interactions would set the dominant energy scale.

Figure 1

Simultaneous quantum degeneracy of ${}^{23}$Na and ${}^{40}$K atoms. (a) and (b), and (d) and (e) are pairs of time-of-flight (TOF) absorption images of a ${}^{23}$Na BEC and a ${}^{40}$K Fermi cloud with different atom number balance. A strong attractive interaction between the two species is observed in (e) as a sharp increase of the central density in the fermionic cloud in the presence of a Bose condensate. (c) and (f) are the center-sliced column density of the fermionic clouds of (b) and (e), respectively. TOF was (a) 11 ms, (b) 7 ms, (d) 17 ms, and (e) 5 ms. Atom numbers are (a) $2.4\times {10}^5$, (b) $2\times {10}^5$, (d) $8.2\times {10}^5$, and (e) $6.7\times {10}^4$.

Figure 2

Evolution of the phase space density (PSD) with atom number ($N$). Blue circles: ${}^{40}$K; red squares: ${}^{23}$Na with ${}^{40}$K; green diamonds: ${}^{23}$Na alone. Open and solid symbols represent the PSD in the magnetic trap and optical trap, respectively.

Figure 3

Feshbach resonances in ${}^{23}$Na-${}^{40}$K, here for ${}^{23}\mathrm{Na}|1,1\rangle +{}^{40}\mathrm{K}|9/2,-5/2\rangle$ collisions. (a) Scattering length from the ABM model. (b) Open-channel threshold energy (black solid line), uncoupled ($s$-wave: dashed lines; $p$-wave: light blue/light gray lines) and coupled molecular states ($s$-wave: red/gray solid lines; $p$-wave: blue/dark gray solid lines). The blue dots denote experimentally measured resonances. The inset shows the energy, relative to threshold, of the molecular state at the wide $s$-wave resonance at 138 G [33]. (c) Experimental loss spectra of ${}^{40}$K in the presence of ${}^{23}$Na. Three $s$-wave resonances and two $p$-wave manifolds are found, the latter resolved in one doublet and one triplet.

Figure 4

Triplet structure of the $p$-wave resonance at 19.1 G for the ${}^{23}\mathrm{Na}|1,1\rangle +{}^{40}\mathrm{K}|9/2,-9/2\rangle$ spin configuration. A phenomenological triple Gaussian fit is applied as a guide to the eyes.