  Using a three-body model with density-dependent contact interaction, we
discuss the root mean square distance between the two valence neutrons in
$^{11}$Li nuclues as a function of the center of mass of the neutrons relative
to the core nucleus $^9$Li. We show that the mean distance takes a pronounced
minimum around the surface of the nucleus, indicating a strong surface
di-neutron correlation. We demonstrate that the pairing correlation plays an
essential role in this behavior. We also discuss the di-neutron structure in
the $^8$He nucleus.
