Pairing correlations in nuclei on the neutron-drip line

Paring correlations in weakly bound nuclei on the edge of the neutron-drip line is studied by use of a three-body model. A density-dependent contact interaction is employed to calculate the ground state of halo nuclei ⁶He and ¹¹Li, as well as a skin nucleus ²⁴O. Dipole excitations in these nuclei are also studied within the same model. We point out that the dineutron-type correlation plays a dominant role in the halo nuclei ⁶He and ¹¹Li, having the coupled spin of the two neutrons S=0, whereas the correlation similar to the BCS type is important in ²⁴O. Contributions of the spin S=1 and S=0 configurations are separately discussed in the low-energy dipole excitations.