Coexistence of BCS- and BEC-like pair structures in halo nuclei
Résumé
We investigate the spatial structure of two-neutron wave function in a Borromean nucleus $^{11}$Li using a three-body model in which two valence neutrons interact with each other by a density-dependent contact force. The behavior of the neutron Cooper pair at different densities is simulated by calculating the two-neutron wave function at several distances between the core nucleus and the center of mass of the two neutrons. We find that the neutron pair wave function in $^{11}$Li has an oscillation at normal density, while it becomes a well localized single peak in the dilute density region around the nuclear surface. These features are in close analogue to the BCS-BEC crossover of the Cooper pair wave function found in the infinite nuclear matter. The present results also provide a unified picture of the di-neutron and the cigar-like configurations in Borromian nuclei as a manifestation of the BCS-BEC crossover phenomenon.