  We investigate the neutron pair transfer modes associated with the low-lying
quadrupole states in neutron-rich Sn isotopes by means of the quasiparticle
random phase approximation based on the Skyrme-Hartree-Fock-Bogoliubov mean
field model. The transition strength of the quadrupole pair-addition mode
feeding the $2_1^+$ state is enhanced in the Sn isotopes with $A \geq 132$. The
transition density of the pair-addition mode has a large spatial extension in
the exterior of nucleus, reaching far to $r\sim 12-13$ fm. The quadrupole
pair-addition mode reflects sensitively a possible increase of the effective
pairing interaction strength in the surface and exterior regions of
neutron-rich nuclei.
