Di-neutron correlation in monopole pair transfer modes near the neutron-separation energy in Sn, Ni, Ca, Zr isotopes

We discuss the monopole pair transfer by means of the Hartree-Fock-Bogoliubov method and the continuum quasiparticle random phase approximation. We investigate the low-lying pair-vibrational state and the pair-rotational ground state having small separation energy in neutron-rich Sn isotopes and Ni, Zr and Ca isotopes. We show that the transfer amplitudes of the 0(+) states are characterized by the coherent superposition of two-quasiparticle configurations up to high orbital angular momenta l similar to 12. It suggests the transfer motion of a di-neutron.