Fermionic pairs – from the surface to the bulk

How does spatial confinement and shell structure influence the formation and character of fermionic pairs in finite, tunable systems? While the BCS-BEC crossover is well-understood in infinite systems, the microscopic structure of pairing in finite systems—where confinement and shell filling compete with interaction-driven correlations—remains less explored.

Using a highly controllable system of few fermionic atoms (6Li) in an optical tweezer, the authors independently tuned the interaction strength (via Feshbach resonance) and the particle number. By employing single-particle-resolved imaging, they measured the density-density correlation function in both real and momentum space. This allowed them to map the pairing behavior across different regimes, ranging from tightly bound dimers to overlapping Cooper pairs, while accounting for the discrete energy levels imposed by the trap.

The study reveals three distinct pairing regimes based on the hierarchy of length scales: the mean interparticle spacing, the pair size, and the system size. In the weakly interacting, confinement-dominated regime, the researchers observed that closed-shell configurations suppress pairing at the trap center, shifting it toward the low-density surface. In contrast, open-shell systems exhibit stronger central pairing. As the interaction strength or particle number increases, the system transitions into a regime where the trap center displays bulk-like Cooper pairing, while the edges retain dimer-like characteristics. The results show that even for very small systems, the local physics begins to resemble the bulk limit, providing a bridge between mesoscopic cold atoms and pairing phenomena in atomic nuclei.

This work provides a microscopic, real-space view of how quantum matter organizes itself under confinement. By resolving the spatial structure of pairs, the findings offer a direct experimental link between the physics of ultracold atoms and the pairing mechanisms found in complex finite systems like superconducting nanostructures and atomic nuclei, where shell effects are known to be critical.