Isoscalar and neutron modes in the E1 spectra of Ni isotopes and the relevance of shell effects and the continuum

Abstract

We study theoretically the electric dipole transitions of even Ni isotopes at lowenergies, using the self-consistent quasiparticle random-phase approximation with the D1S Gogny interaction and a continuum-RPA model with the SLy4 Skyrme force. We analyze isoscalar states, isovector states, and the dipole polarizability. We define a reference value for the polarizability to remove a trivial dependence on the mass number. We compare our results with data and other calculations, with a focus on collective states, shell effects, and threshold transitions. Our results support the presence of a strong isoscalar transition, with little or moderate E1 strength, as a universal feature of ordinary nuclei. In moderately neutron-rich Ni isotopes, namely Ni-68 and neighboring isotopes, this transition is found bimodal due to couplings with surface neutrons. An adequate treatment of the continuum states appears essential for describing suprathreshold E1 strength, especially beyond Ni-68. Very exotic isotopes (N > 50) are found highly polarizable, with practically all their E1 strength in the continuum. The dipole polarizability and the neutron-skin thickness are influenced by shell structure in different ways, so they can appear anticorrelated. A comparison with existing results for lighter (Ca) and heavier (Sn) nuclei suggests that the so-called pygmy dipole strength is influenced strongly by shell effects and that, partly for that reason, its isospin structure depends on the mass region.