Dipole-Mode Spectrum and Hydrodynamic Crossover in a Resonantly Interacting Two-Species Fermion Mixture

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, as 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 $^{161}$Dy and $^{40}$K 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 against experimental imperfections. It provides an accurate measure for the interspecies drag effect, which generalizes the concept of spin drag explored in other experiments. We characterize the interspecies drag locally in terms of a microscopic friction coefficient and we discuss its unitarity-limited universal behavior on top of the resonance.

💡 Research Summary

The study presented in this paper investigates the complex dynamics of an ultracold mixture of two different species of fermions, $^{161}$Dy and $^{4뮬40}$K, focusing on the transition between collisionless and hydrodynamic regimes. By utilizing Feshbach resonances, the researchers were able to precisely tune the interspecies interactions, allowing them to observe how the system’s collective behavior evolves as it moves toward a strongly correlated state.

The core of the experimental investigation lies in the analysis of the dipole-mode spectrum. The researchers monitored the frequency and damping rates of these modes across a wide range of interaction strengths. A significant finding is the behavior of the first oscillating mode, which persists throughout the entire interaction range but undergoes dramatic changes in both frequency and damping as the system enters the deep hydrodynamic regime. This provides clear evidence of the emergence of hydrodynamic collective behavior in these quantum gases.

Furthermore, the study reveals a fascinating phenomenon regarding the second oscillating mode: it splits into two distinct modes characterized by purely exponential damping. One of these modes is particularly noteworthy because it is highly robust against experimental imperfections, making it an exceptionally reliable probe for measuring the interspecies drag effect. This drag effect, which generalizes the well-known concept of spin drag, allows for a precise characterization of the microscopic friction between the two atomic species.

By comparing experimental results with a theoretical model that accounts for the motion of the mass centers of both species, the authors successfully decoupled the contributions of mean-field interactions and friction. The research provides a microscopic description of the friction coefficient and discusses its universal behavior near the unitarity limit, where the scattering cross-section reaches its maximum possible value. This work not only advances our understanding of hydrodynamic crossovers in quantum mixtures but also establishes a new, highly accurate method for studying interspecies momentum transfer and friction in strongly interacting Fermi gases.