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 $^{6}\mathrm{He}$ and $^{11}\mathrm{Li}$, as well as a skin nucleus $^{24}\mathrm{O}$. 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 $^{6}\mathrm{He}$ and $^{11}\mathrm{Li}$, having the coupled spin of the two neutrons $S=0$, whereas the correlation similar to the BCS type is important in $^{24}\mathrm{O}$. Contributions of the spin $S=1$ and $S=0$ configurations are separately discussed in the low-energy dipole excitations.