Novel superfluids in ultracold fermionic mixtures
The intricate mechanisms leading to superfluidity and superconductivity have fascinated researchers for many decades. The pairing of fermionic constituents is at the heart of a rich class of related phenomena in quantum matter. The understanding of unconventional regimes of superfluidity, such as broken-symmetry systems with finite-momentum pairs, poses great challenges for theory and experiment.

Our experimental approach is based on a new ultracold model system, which has become available very recently, following a breakthrough in our laboratory. We combine two fermionic isotopes with different masses (Dy-161 and K-40) to create a quantum-degenerate Fermi-Fermi mixture. The system features the key ingredients required for creating novel strongly interacting superfluids: mass imbalance, tunability of interactions, and collisional stability. At present, our Dy-K system represents the only experimentally available Fermi-Fermi mixture that offers this combination of properties.

The key point is that, according to theoretical predictions, mass imbalance favors unconventional regimes of superfluidity. In contrast to the widely investigated ultracold fermionic spin mixtures (systems with equal masses), the regions in the phase diagram for unconventional phases are much larger and appear at temperatures that can be realistically achieved in experiments.

We will realize strongly interacting Fermi-Fermi superfluids and investigate the various regimes of superfluidity with the ultimate prospect to demonstrate symmetry-broken phases. For this purpose, we will generalize the methods developed for spin mixtures, and we will develop new species-selective optical traps. We will carry out our main research on the Dy-K system, but we will also push the limits of mass-imbalanced systems by extending our studies to a new system with extreme mass ratio (mixture of Dy-161 with Li-6). Our work will lead to a new generation of experiments on ultracold fermions.