Surface-enhanced pair transfer amplitude in quadrupole states of neutron-rich Sn isotopes

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 21+ state is enhanced in the Sn isotopes with A≥132. The transition density of the pair-addition mode has a large spatial extension in the exterior of the nucleus, reaching to r∼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.