Observation of a Strong Atom-Dimer Attraction in a Mass-Imbalanced Fermi-Fermi Mixture

We investigate a mixture of ultracold fermionic ${}^{40}\mathrm{K}$ atoms and weakly bound ${}^6\mathrm{Li}{}^{40}\mathrm{K}$ dimers on the repulsive side of a heteronuclear atomic Feshbach resonance. By radio-frequency spectroscopy we demonstrate that the normally repulsive atom-dimer interaction is turned into a strong attraction. The phenomenon can be understood as a three-body effect in which two heavy ${}^{40}\mathrm{K}$ fermions exchange the light ${}^6\mathrm{Li}$ atom, leading to attraction in odd partial-wave channels (mainly $p$ wave). Our observations show that mass imbalance in a fermionic system can profoundly change the character of interactions as compared to the well-established mass-balanced case.

Figure 1

Interaction between ${}^{40}\mathrm{K}$ atoms and ${}^6\mathrm{Li}{}^{40}\mathrm{K}$ dimers near the 155 G interspecies FR. (a) Total interaction potentials as a function of the distance $R$ between the two K atoms for the $s$, $p$, and $d$ channels (dashed curves with labels $s^{\prime }$, $p^{\prime }$, $d^{\prime }$ refer to the unmodified centrifugal barriers). Here we have chosen a magnetic detuning of $B-B_0=-16\mathrm{mG}$, corresponding to an $s$-wave scattering length of $a=3500{\mathrm{a}}_0$ and to a dimer binding energy of $E_{\mathrm{b}}/k_{\mathrm{B}}=600\mathrm{nK}$. (b) Real part of the forward-scattering amplitude $f(0)$ as a function of the collision energy $E_{\text{coll}}$ (solid line) in comparison with the $s$-wave contribution (dashed line). (c) Same as in (b), but as a function of the magnetic detuning $B-B_0$ for a fixed collision energy $E_{\text{coll}}/k_{\mathrm{B}}=350\mathrm{nK}$. The dotted line indicates the dimer breakup threshold, $E_{\text{coll}}=E_{\mathrm{b}}$.

Figure 2

Sample rf spectra taken at $B-B_0=-20\mathrm{mG}$ at $T_{\text{eff}}=232\mathrm{nK}$. The red diamonds (blue circles) show data recorded using method A (B). For reference, the gray triangles show data recorded in the absence of dimers together with a Gaussian fit (gray line). Inset: Spectrum at $-17\mathrm{mG}$ over an extended frequency range. The molecular dissociation signal (open symbols), recorded with $30\times$ increased rf power, is clearly separated from the atomic peak (filled symbols).

Figure 3

Widths (blue triangles) and peak shifts (red circles) extracted from the rf spectra as a function of the magnetic field detuning $B-B_{\mathrm{0}}$ for the three different values of $T_{\text{eff}}$. The lines are the corresponding theoretical predictions. To account for fluctuations in the dimer number of different spectra, the widths and peak shifts are scaled to a dimer number of 15 000, which is typical for all spectra.

Figure 4

Real part of the atom-dimer forward-scattering amplitude as a function of the atom-atom scattering length $a$ for the three different values of $T_{\text{eff}}$. The symbols and the lines show the data and the theoretical predictions from Fig. 3. For comparison, the dashed lines indicate the respective $s$-wave contributions. The theoretical lines stop at $k_{\mathrm{B}}{T}_{\text{eff}}=E_{\mathrm{b}}/2$.