1 Di-neutron correlation in soft octupole excitations of neutron-rich Ni isotopes beyond N = 50

We investigate low-lying octupole response of neutron-rich Ni isotopes beyond the N=50 shell closure using the Skyrme-Hartree-Fock-Bogoliubov mean-fields and the continuum quasi-particle random phase approximation. Performing detailed numerical analyses employing the Skyrme parameter set SLy4 and a density-dependent delta interaction of the mixed type, we show that a neutron mode emerges above the neutron separation energy as a consequence of the weak binding of neutrons and it exhibits strong influences of the di-neutron correlation. The di-neutron correlation, a spatial pair correlation with small correlation length among neutrons, has been one of the central themes of the physics of twoneutron halo nuclei such as 11 Li. 1)–7) Although an affirmative experimental signature of the neutron spatial correlation in 11 Li is obtained rather recently, 8) many theoretical