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arXiv:2506.23318 · Communications Physics

Tunable field-linked s-wave interactions in dipolar fermi mixtures

Jing-Lun Li, Georgios M. Koutentakis, Mateja Hrast, Mikhail Lemeshko, Andreas Schindewolf, Ragheed Alhyder

Published Apr 14, 2026• 2 citations
arXivHTMLPDF

Abstract

Abstract Spin mixtures of degenerate fermions are a cornerstone of quantum many-body physics, enabling superfluidity, polarons, and rich spin dynamics through s -wave scattering resonances. Combining them with strong, long-range dipolar interactions provides highly flexible control schemes promising even more exotic quantum phases. Recently, microwave shielding gave access to spin-polarized degenerate samples of dipolar fermionic molecules, where tunable p -wave interactions were enabled by field-linked resonances available only by compromising the shielding (due to experimental limitations). Here, we study the scattering properties of a fermionic dipolar spin mixture and show that a universal s -wave resonance is readily accessible without compromising the shielding. We develop a universal description of the tunable s -wave interaction and weakly bound tetratomic states based on the microwave-field parameters. The s -wave resonance paves the way to stable, controllable and strongly-interacting dipolar spin mixtures of deeply degenerate fermions and supports favorable conditions to reach this regime via evaporative cooling.

DOI
10.1038/s42005-026-02578-8

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Abstract Spin mixtures of degenerate fermions are a cornerstone of quantum many-body physics, enabling superfluidity, polarons, and rich spin dynamics through s -wave scattering resonances. Combining them with strong, long-range dipolar interactions provides highly flexible control schemes promising even more exotic quantum phases. Recently, microwave shielding gave access to spin-polarized degenerate samples of dipolar fermionic molecules, where tunable p -wave interactions were enabled by field-linked resonances available only by compromising the shielding (due to experimental limitations). Here, we study the scattering properties of a fermionic dipolar spin mixture and show that a universal s -wave resonance is readily accessible without compromising the shielding. We develop a universal description of the tunable s -wave interaction and weakly bound tetratomic states based on the microwave-field parameters. The s -wave resonance paves the way to stable, controllable and strongly-interacting dipolar spin mixtures of deeply degenerate fermions and supports favorable conditions to reach this regime via evaporative cooling.
Long-range anisotropic dipole-dipole interactions between ultracold polar molecules are predicted to drive exotic quantum phases, yet direct many-body signatures of these interactions in degenerate Fermi gases have remained elusive. Here, we report the observation of an interaction-induced controlled deformation of the Fermi surface, providing a clear many-body signature in a deeply degenerate Fermi gas of 23Na40K^{23}\text{Na}^{40}\text{K}23Na40K molecules. Using double microwave (MW) shielding, we prepare 8×1038 \times 10^38×103 molecules at 0.23(1)0.23(1)0.23(1) times the Fermi temperature, achieving a three-fold suppression of inelastic losses compared to single MW shielding while preserving strong elastic dipolar scattering. We observe Fermi surface deformations of up to 7 %7\,\%7%, more than two times larger than those observed in magnetic atoms, despite operating at two orders of magnitude lower densities. Crucially, we demonstrate continuous tuning of the interaction potential from axial U(1) to biaxial C2_{2}2​ symmetry, directly imprinting this geometry onto the Fermi surface. We find excellent agreement between our experimental results and parameter-free Hartree-Fock theory. These results establish MW-shielded polar molecules as a highly tunable platform for exploring strongly correlated dipolar Fermi matter and offer a promising path towards topological superfluidity.
The recent experimental progress in realizing Bose-Fermi mixtures and dipolar quantum gases has created the exciting possibility of creating dipolar Bose-Fermi mixtures with intriguing and unique properties. In a dipolar condensate, the dipole-dipole interaction represents a control knob inaccessible to nondipolar Bosons. Thus, mixing dipolar bosons with fermions may open up new possibilities for creating superfluids with unconventional pairings. We consider a mixture of a spin-polarized Fermi gas and a dipolar Bose-Einstein condensate in a three-dimensional setting in which s-wave scattering between fermions and the quasiparticles of the dipolar condensate can result in an effective attractive Fermi-Fermi interaction anisotropic in nature and tunable by the dipolar interaction. We develop a procedure which allows us to determine the parameters such as densities and scattering lengths that are required to achieve optimal critical temperature before the system starts to phase separate. We perform a systematic investigation, finding that a superfluid with a critical temperature, that is orders of magnitude higher than achievable in nondipolar mixtures, can be realized in dipolar mixtures at experimentally accessible densities and scattering lengths before the system phase separates.
Long-range anisotropic dipole-dipole interactions between ultracold polar molecules are predicted to drive exotic quantum phases, yet direct many-body signatures of these interactions in degenerate Fermi gases have remained elusive. Here, we report the observation of an interaction-induced controlled deformation of the Fermi surface, providing a clear many-body signature in a deeply degenerate Fermi gas of 23Na40K^{23}\text{Na}^{40}\text{K}23Na40K molecules. Using double microwave (MW) shielding, we prepare 8×1038 \times 10^38×103 molecules at 0.23(1)0.23(1)0.23(1) times the Fermi temperature, achieving a three-fold suppression of inelastic losses compared to single MW shielding while preserving strong elastic dipolar scattering. We observe Fermi surface deformations of up to 7 %7\,\%7%, more than two times larger than those observed in magnetic atoms, despite operating at two orders of magnitude lower densities. Crucially, we demonstrate continuous tuning of the interaction potential from axial U(1) to biaxial C2_{2}2​ symmetry, directly imprinting this geometry onto the Fermi surface. We find excellent agreement between our experimental results and parameter-free Hartree-Fock theory. These results establish MW-shielded polar molecules as a highly tunable platform for exploring strongly correlated dipolar Fermi matter and offer a promising path towards topological superfluidity.
The topic of the present lecture notes are two-species quantum mixtures composed of a deeply degenerate Fermi gas and a second component, the latter being fermionic or bosonic. A key ingredient is the possibility to tune the sss-wave interaction between the different species by means of magnetically controlled Feshbach resonances, which allow us to investigate regimes of strong interactions. In two case studies, we review our experiments on mixtures of 6^66Li fermions with fermionic 40^{40}40K or bosonic 41^{41}41K atoms and on mixtures of fermionic 161^{161}161Dy with 40^{40}40K atoms. We cover various topics of fermionic quantum many-body physics, ranging from impurity physics and quasiparticles over phase separation to the formation of ultracold molecules and progress towards novel superfluids.
Abstract The control of molecules is key to the investigation of quantum phases, in which rich degrees of freedom can be used to encode information and strong interactions can be precisely tuned1. Inelastic losses in molecular collisions2–5, however, have greatly hampered the engineering of low-entropy molecular systems6. So far, the only quantum degenerate gas of molecules has been created via association of two highly degenerate atomic gases7, 8. Here we use an external electric field along with optical lattice confinement to create a two-dimensional Fermi gas of spin-polarized potassium–rubidium (KRb) polar molecules, in which elastic, tunable dipolar interactions dominate over all inelastic processes. Direct thermalization among the molecules in the trap leads to efficient dipolar evaporative cooling, yielding a rapid increase in phase-space density. At the onset of quantum degeneracy, we observe the effects of Fermi statistics on the thermodynamics of the molecular gas. These results demonstrate a general strategy for achieving quantum degeneracy in dipolar molecular gases in which strong, long-range and anisotropic dipolar interactions can drive the emergence of exotic many-body phases, such as interlayer pairing and p-wave superfluidity.
We have created quantum degenerate Fermi gases of 6Li^{6}\mathrm{Li}6Li atoms at unprecedented high densities exceeding $1/{\ensuremath{\lambda}\phantom{\rule{-0.5em}{0ex}}\ensuremath{-}}^{3}$ (where $\ensuremath{\lambda}$ is the wavelength of resonant light). In this regime, new optical properties are predicted. Spin-polarized Fermi gases are usually regarded as noninteracting due to the weakness of ppp-wave interactions. Here we study the properties of ``ppp-wave matter,'' where elastic and inelastic ppp-wave collisions determine the dynamics of the system. We characterize thermalization, evaporative cooling, and inelastic two-body and three-body collisions. PPP-wave dipolar relaxation creates a metastable mixture of the lowest and highest hyperfine states.