Coexistence of BCS- and BEC-Like pair structures in halo nuclei

We investigate the spatial structure of the two-neutron wave function in the Borromean nucleus Li-11, using a three-body model of Li-9+n+n, which includes many-body correlations stemming from the Pauli principle. The behavior of the neutron pair at different densities is simulated by calculating the two-neutron wave function at several distances between the core nucleus Li-9 and the center of mass of the two neutrons. With this representation, a strong concentration of the neutron pair on the nuclear surface is for the first time quantitatively established for neutron-rich nuclei. That is, the neutron pair wave function in Li-11 has an oscillatory behavior at normal density, while it becomes a well-localized single peak in the dilute density region around the nuclear surface. We point out that these features qualitatively correspond to the BCS- and BEC-like structures of the pair wave function found in infinite nuclear matter.