Spatially extended coherence induced by pairing correlation in low-frequency vibrational excitations of neutron drip line nuclei

Role of pairing correlation for emergence of low-frequency vibrational excitations in neutron drip line nuclei is discussed paying special attention to neutrons with small orbital angular momentum l. Self-consistent pairing correlation in the Hartree-Fock-Bogoliubov (HFB) theory causes the change of the spatial structure of the quasiparticle wave functions; "the pairing anti-halo effect" in the lower component and "the broadening effect" in the upper component. The resultant spatial distribution of the two-quasiparticle states among low-l neutrons, "the broad localization", brings about qualitatively new aspects, especially the large transition strength of the low-frequency vibrational excitations in nuclei close to the neutron drip line. By performing HFB plus quasiparticle random phase approximation (QRPA) calculation for the first 2+ states in neutron rich Ni isotopes, the unique role of self-consistent pairing correlation is pointed out.