We have identified a low-field Feshbach resonance in the Fermi-Fermi mixture of Dy and K, which was previously unknown. The magnetically tuned resonance turned out to be very beneficial for accurate interaction control in this mixture, and will serve as an important tool in our future experiments. We have thoroughly characterized the resonance by interspecies thermalization, binding energy measurements, and magnetic moment spectroscopy, and thus gained a detailed understanding of elastic and inelastic interactions near this resonance.
We have demonstrated the formation of ultracold weakly bound “Feshbach” molecules of Dy and K and prepared a pure molecular sample in an optical dipole trap. The DyK molecules are bosons and, with measured phase-space densities close to one, conditions are close to molecular Bose-Einstein condensation. We identified an unexpected loss mechanism, which limits the lifetime of the molecular sample in the optical trap. The trap light itself can induce losses, which we interpret as a consequence of the extremely high density of molecular states in the DyK molecules. We have also shown that these losses can be suppressed by choosing the particular wavelength of the optical trap.
For probing hydrodynamic behavior, we have studied the two-species dipole mode spectrum in the Dy-K mixture. By variation of the interaction strength across the Feshbach resonance, we investigated the resonant crossover from collisionless to hydrodynamic behavior. We observed a fast damping mode, which was not seen in earlier experiments and which provides direct information on the interspecies friction. A universal transport coefficient can be derived from these measurements.
In our experiments on Li-K mixtures, we addressed a long-standing question concerning the polaronic regime, where a minority of particles (K atoms) is immersed in a large Fermi sea of majority atoms (Li). We verified essential predictions of Landau’s Fermi-liquid theory concerning the sign (i.e. the attractive or repulsive nature) of mediated interactions. Changing K isotopes (from fermionic to bosonic), we could also demonstrate that this sign depends on the quantum statistics of the impurity atoms.
Various technological developments have been made: Shaping optical potentials by digital-mirror devices, pushing optical dipole traps further into the infrared, realizing species-selective momentum changes by Raman transitions, and improved atomic beam sources.
