Pairing correlations in nuclei on the neutron-drip line

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