Extended 9Li+n+n three-body model of 11Li with a pairing correlation in 9Li

We investigate the binding mechanism of 11Li on the basis of an extended three-body model of 9Li+n+n. In the model, we take into account the pairing correlation of p-shell neutrons in 9Li, in addition to that of valence neutrons outside the 9Li nucleus, and solve the coupled-channel two- and three-body problems of 10Li and 11Li, respectively. The results show that the degrees of freedom of the pairing correlation in 9Li play an important role in the structure of 10Li and 11Li. In 10Li, the pairing correlation in 9Li produces a so-called pairing-blocking effect due to the presence of a valence neutron, which causes the s- and p-wave neutron orbits to become energetically degenerate. In 11Li, contrastingly, the pairing-blocking effect is overwhelmed by the core-n interaction due to the two degrees of freedom of the two valence neutrons surrounding 9Li, and as a result, the ground state is dominated by the p-shell closed configuration and does not exhibit spatial extension with a large r.m.s. radius. These results indicate that the pairing correlation is realized differently in odd- and even-neutron systems of 10Li and 11Li. In addition, we further improve the tail part of the 9Li-n interaction, which yields reasonable reproductions of the observed large r.m.s. radius in 11Li. (author)