Dineutron correlations in nuclear surface

Two-neutron correlation in quasi two-dimensional (2D) neutron matter is studied by means of the BCS theory to understand formation of nn pairs in nuclear surface of neutron-rich nuclei. The spin-zero nn pair correlation in low density neutron systems confined in an infinite slab is investigated in a simplified model that neutron motion of one direction is frozen. It is found that, when the slab is thin enough, the nn pairing gap enhances and the size shrinking of nn Cooper pair occurs at finite low-density region in the quasi-2D system.