We report on the realization of a Fermi-Fermi mixture of ultracold atoms that combines mass imbalance, tunability, and collisional stability. In an optically trapped sample of $^{161}$ Dy and $^{40}$ K, we identify a broad Feshbach resonance centered at a magnetic field of 218 $\,$ G. In the strongly interacting regime, we demonstrate hydrodynamic behavior in the expansion after release from the trap. Lifetime studies on resonance show that the fermionic nature of the system suppresses inelastic few-body processes by several orders of magnitude. The resonant mixture opens up intriguing perspectives for studies on novel states of strongly correlated fermions with mass imbalance.We report on the realization of a Fermi-Fermi mixture of ultracold atoms that combines mass imbalance, tunability, and collisional stability. In an optically trapped sample of $^{161}$Dy and $^{40}$K, we identify a broad Feshbach resonance centered at a magnetic field of 218$\,$G. In the strongly interacting regime, we demonstrate hydrodynamic behavior in the expansion after release from the trap. Lifetime studies on resonance show that the fermionic nature of the system suppresses inelastic few-body processes by several orders of magnitude. The resonant mixture opens up intriguing perspectives for studies on novel states of strongly correlated fermions with mass imbalance.

Sep 8, 20190 citations

Observation of low-field Feshbach resonances between $^{161}$ Dy and $^{40}$ KObservation of low-field Feshbach resonances between $^{161}$Dy and $^{40}$K

Zhu-Xiong Ye, Alberto Canali, Elisa Soave +5 more

We report on the observation of Feshbach resonances at low magnetic field strength (below 10 G) in the Fermi-Fermi mixture of $^{161}$ Dy and $^{40}$ K. We characterize five resonances by measurements of interspecies thermalization rates and molecular binding energies. As a case of particular interest for applications, we consider a resonance near 7.29 G, which combines accurate magnetic tunability and access to the universal regime of interactions with experimental simplicity. We show that lifetimes of a few 100 ms can be achieved for the optically trapped, resonantly interacting mixture. We also demonstrate the hydrodynamic expansion of the mixture in the strongly interacting regime and the formation of DyK Feshbach molecules. Our work opens up new experimental possibilities in view of mass-imbalanced superfluids and related phenomena.We report on the observation of Feshbach resonances at low magnetic field strength (below 10 G) in the Fermi-Fermi mixture of $^{161}$Dy and $^{40}$K. We characterize five resonances by measurements of interspecies thermalization rates and molecular binding energies. As a case of particular interest for applications, we consider a resonance near 7.29 G, which combines accurate magnetic tunability and access to the universal regime of interactions with experimental simplicity. We show that lifetimes of a few 100 ms can be achieved for the optically trapped, resonantly interacting mixture. We also demonstrate the hydrodynamic expansion of the mixture in the strongly interacting regime and the formation of DyK Feshbach molecules. Our work opens up new experimental possibilities in view of mass-imbalanced superfluids and related phenomena.

arXiv:2207.03407Jul 7, 20220 citations

Observation of low-field Feshbach resonances between Dy161 and K40

Zhu-Xiong Ye, Alberto Canali, Elisa Soave +5 more

The authors report the observation of Feshbach resonances in the Fermi-Fermi mixture of ultracold ${}^{161}$ Dy and ${}^{40}$ K. The authors show experimental evidence that Dy-K offers a Feshbach resonance at 7.29 G with properties that could make the study of resonantly interacting mass-imbalanced Fermi-Fermi mixtures possible. In addition, they demonstrate the formation of DyK Feshbach molecules.The authors report the observation of Feshbach resonances in the Fermi-Fermi mixture of ultracold ${}^{161}$Dy and ${}^{40}$K. The authors show experimental evidence that Dy-K offers a Feshbach resonance at 7.29 G with properties that could make the study of resonantly interacting mass-imbalanced Fermi-Fermi mixtures possible. In addition, they demonstrate the formation of DyK Feshbach molecules.

Physical Review AOct 17, 202215 citations

Dipole-mode spectrum and hydrodynamic crossover in a resonantly interacting two-species fermion mixture

Zhu-Xiong Ye, Alberto Canali, Chun-Kit Wong +3 more

Ultracold quantum-gas mixtures of fermionic atoms with resonant control of interactions offer a unique test-bed to explore few- and many-body quantum states with unconventional properties. The emergence of such strongly correlated systems, for instance, symmetry-broken superfluids, is usually accompanied by hydrodynamic collective behavior. Thus experimental progress in this field naturally requires a deep understanding of hydrodynamic regimes. Here, we report on experiments employing a tunable Fermi-Fermi mixture of Dy 161 and K 40 near quantum degeneracy. We investigate the full spectrum of dipole modes across a Feshbach resonance and characterize the crossover from collisionless to deep hydrodynamic behavior in measurements of frequencies and damping rates. We compare our results with a theoretical model that considers the motion of the mass centers of the two species, and we identify the contributions of friction and mean-field interaction. We show that one oscillating mode exists over the whole range of interactions, exhibiting striking changes of frequency and damping in the deep hydrodynamic regime. We observe the second oscillating mode to split into two purely exponential damping modes. One of these exponential modes shows very fast damping, faster than any other relevant timescale, and is largely insensitive to experimental imperfections. It provides an accurate measure for the interspecies drag effect, which generalizes the concept of spin drag explored in other experiments. We finally characterize the interspecies drag in terms of a microscopic friction coefficient, and we discuss its unitarity-limited universal behavior on top of the resonance.Ultracold quantum-gas mixtures of fermionic atoms with resonant control of interactions offer a unique test-bed to explore few- and many-body quantum states with unconventional properties. The emergence of such strongly correlated systems, for instance, symmetry-broken superfluids, is usually accompanied by hydrodynamic collective behavior. Thus experimental progress in this field naturally requires a deep understanding of hydrodynamic regimes. Here, we report on experiments employing a tunable Fermi-Fermi mixture of Dy 161 and K 40 near quantum degeneracy. We investigate the full spectrum of dipole modes across a Feshbach resonance and characterize the crossover from collisionless to deep hydrodynamic behavior in measurements of frequencies and damping rates. We compare our results with a theoretical model that considers the motion of the mass centers of the two species, and we identify the contributions of friction and mean-field interaction. We show that one oscillating mode exists over the whole range of interactions, exhibiting striking changes of frequency and damping in the deep hydrodynamic regime. We observe the second oscillating mode to split into two purely exponential damping modes. One of these exponential modes shows very fast damping, faster than any other relevant timescale, and is largely insensitive to experimental imperfections. It provides an accurate measure for the interspecies drag effect, which generalizes the concept of spin drag explored in other experiments. We finally characterize the interspecies drag in terms of a microscopic friction coefficient, and we discuss its unitarity-limited universal behavior on top of the resonance.

arXiv:2502.05006Physical Review ResearchJun 13, 20252 citations

Optically trapped Feshbach molecules of fermionic Dy161 and K40

Elisa Soave, Alberto Canali, Zhu-Xiong Ye +3 more

A pure sample of bosonic Feshbach molecules composed of fermionic Dy-161 and K-40 atoms has been prepared in an optical trap, with a phase-space density of about 0.1. Investigation of the molecular cloud decay in the presence of 1064-nm trapping light points to a one-photon coupling to electronically excited states as the main factor limiting the sample lifetime. This observation, together with a refined understanding of the molecule properties and behavior as a function of the magnetic field, offers crucial insights and experimental progress toward achieving quantum-degenerate samples of DyK molecules and novel superfluids based on mass-imbalanced fermion mixtures.A pure sample of bosonic Feshbach molecules composed of fermionic Dy-161 and K-40 atoms has been prepared in an optical trap, with a phase-space density of about 0.1. Investigation of the molecular cloud decay in the presence of 1064-nm trapping light points to a one-photon coupling to electronically excited states as the main factor limiting the sample lifetime. This observation, together with a refined understanding of the molecule properties and behavior as a function of the magnetic field, offers crucial insights and experimental progress toward achieving quantum-degenerate samples of DyK molecules and novel superfluids based on mass-imbalanced fermion mixtures.

Physical Review ResearchAug 18, 202311 citations

Broad Feshbach Resonance with a Large Background Scattering Length in a Fermionic Atom-Molecule Mixture

Zhen Su, Tong-Hui Shou, Huan Yang +6 more

We report the observation of a broad magnetic Feshbach resonance with a large background scattering length in an ultracold fermionic mixture of ^{23}Na^{40}K molecules and ^{40}K atoms, with both species prepared in their lowest hyperfine states. The Feshbach resonance is characterized by measuring resonantly enhanced loss rates and elastic scattering cross sections via cross-species thermalization. The large background scattering length can drive the atom-molecule mixture into the hydrodynamic regime when the magnetic field is far from the resonance. We observe that the center-of-mass motions of the atoms and molecules are phase locked and oscillate with a common frequency due to hydrodynamic drag effects. This broad atom-molecule Feshbach resonance with its large background scattering length opens up a new avenue toward studying strongly interacting fermionic gases with mass imbalance.We report the observation of a broad magnetic Feshbach resonance with a large background scattering length in an ultracold fermionic mixture of ^{23}Na^{40}K molecules and ^{40}K atoms, with both species prepared in their lowest hyperfine states. The Feshbach resonance is characterized by measuring resonantly enhanced loss rates and elastic scattering cross sections via cross-species thermalization. The large background scattering length can drive the atom-molecule mixture into the hydrodynamic regime when the magnetic field is far from the resonance. We observe that the center-of-mass motions of the atoms and molecules are phase locked and oscillate with a common frequency due to hydrodynamic drag effects. This broad atom-molecule Feshbach resonance with its large background scattering length opens up a new avenue toward studying strongly interacting fermionic gases with mass imbalance.

arXiv:2511.06772Physical Review LettersNov 6, 20252 citations

Strongly Interacting Fermi-Fermi Mixture of

^{161}

Dy and

^{40}

We report on the realization of a Fermi-Fermi mixture of ultracold atoms that combines mass imbalance, tunability, and collisional stability. In an optically trapped sample of

^{161}

Dy and

^{40}

K, we identify a broad Feshbach resonance centered at a magnetic field of 218

\,

G. In the strongly interacting regime, we demonstrate hydrodynamic behavior in the expansion after release from the trap. Lifetime studies on resonance show that the fermionic nature of the system suppresses inelastic few-body processes by several orders of magnitude. The resonant mixture opens up intriguing perspectives for studies on novel states of strongly correlated fermions with mass imbalance.

Observation of low-field Feshbach resonances between

^{161}

Dy and

^{40}

We report on the observation of Feshbach resonances at low magnetic field strength (below 10 G) in the Fermi-Fermi mixture of

^{161}

Dy and

^{40}

K. We characterize five resonances by measurements of interspecies thermalization rates and molecular binding energies. As a case of particular interest for applications, we consider a resonance near 7.29 G, which combines accurate magnetic tunability and access to the universal regime of interactions with experimental simplicity. We show that lifetimes of a few 100 ms can be achieved for the optically trapped, resonantly interacting mixture. We also demonstrate the hydrodynamic expansion of the mixture in the strongly interacting regime and the formation of DyK Feshbach molecules. Our work opens up new experimental possibilities in view of mass-imbalanced superfluids and related phenomena.

The authors report the observation of Feshbach resonances in the Fermi-Fermi mixture of ultracold

{}^{161}

Dy and

{}^{40}

K. The authors show experimental evidence that Dy-K offers a Feshbach resonance at 7.29 G with properties that could make the study of resonantly interacting mass-imbalanced Fermi-Fermi mixtures possible. In addition, they demonstrate the formation of DyK Feshbach molecules.