Researchers Find Pair Correlations Shift From Surface to Bulk in Few-Atom Systems

Researchers led by Sandra Brandstetter of teh Physikalisches Institut der Universit¨at Heidelberg and colleague form Technische Universit¨at Darmstadt, Lund University and Aarhus University, have imaged pair correlations in a finite, tunable system of few fermionic atoms, revealing a surprising shift in pairing behavior. The team used single-particle resolution imaging to observe pair correlations, a level of detail previously inaccessible. Their work demonstrates that closed-shell configurations suppress correlations in the high-density center of an atomic trap, with pairing instead occurring at the low-density surface. “By resolving where pairs form and how their character changes from localized dimers to overlapping Cooper pairs, our measurements provide a microscopic view of pairing in finite fermionic matter,” the authors write, connecting these findings to phenomena in nuclei and superconducting nanostructures. Even with surprisingly few atoms, increasing interaction strength restores a “locally bulk-like Cooper-pair profile” in the trap center, alongside dimer-like correlations at the edge.

The behavior of paired fermions, fundamental to phenomena ranging from superconductivity to nuclear structure, exhibits a surprising sensitivity to confinement and particle number. Their work reveals that the familiar BCS-BEC crossover, describing the evolution from broad Cooper pairs to tight dimers, is significantly altered within confined, finite systems. Specifically, closed-shell configurations within the atomic trap suppress correlations at the high-density center, a counterintuitive result given typical pairing expectations. Instead of central pairing, the team observed that pairing predominantly occurs at the low-density surface when the system is weakly interacting and exhibits closed-shell configurations. Open-shell systems, however, demonstrate substantially stronger pairing in the trap’s center. This shift in pairing behavior is notable, as the edge of the trap simultaneously retains dimer-like correlations. This detailed understanding of finite fermionic systems promises to refine models of pairing across diverse areas of physics.

Researchers are no longer limited to studying these pairings in bulk materials; single-particle resolution imaging allows direct observation of how these correlations form in systems containing only a few atoms. This level of detail was previously inaccessible, offering a new window into the microscopic origins of pairing in confined environments. The work reveals a surprising interplay between confinement and atomic shell structure, fundamentally altering where pairing occurs within an atomic trap. Closed-shell configurations, where these configurations are filled, actively suppress pair correlations in the high-density center of the trap, and pairing instead concentrates at the lower-density periphery. The study reports that “in the weakly interacting, confinement-dominated regime, closed-shell configurations suppress correlations in the high-density trap center.” However, open-shell systems exhibit significantly stronger pairing in the trap’s central region, demonstrating that shell filling is a critical factor. Remarkably, even remarkably small systems can rapidly transition to a “locally bulk-like Cooper-pair profile” when interaction strength or particle number increases, while the outer edges maintain dimer-like correlations.

Researchers working with ultracold Fermi gases are revealing how pairing behavior shifts in systems with just a few atoms. The team led by Sandra Brandstetter utilized single-particle resolution imaging to image pair correlations, a feat previously unattainable in such small-scale systems. This transition is notable; the outer edges of the trap simultaneously retain dimer-like correlations, indicating a coexistence of different pairing mechanisms within the same system. These findings connect the behavior of these mesoscopic cold atoms to pairing phenomena observed in atomic nuclei and superconducting nanostructures, offering a new perspective on quantum phenomena across diverse materials. The ability to tune the interaction strength and particle number allows for precise control over the pairing process, furthering understanding of the BCS-BEC crossover in confined systems.