Low-energy monopole strength in exotic nickel isotopes

Low-energy strength is predicted for the isoscalar monopole response of neutron-rich Ni isotopes, in calculations performed using the microscopic Skyrme HF + RPA and relativistic RHB + RQRPA models. Both models, although based on different energy density functionals, predict the occurrence of pronounced monopole states in the energy region between 10 and 15 MeV, well separated from the isoscalar giant monopole resonance. The analysis of transition densities and corresponding particle-hole configurations shows that these states represent almost-pure neutron single hole-particle excitations. Even though their location is not modified with respect to the corresponding unperturbed states, their strength is considerably enhanced by the residual interaction. The theoretical analysis predicts the gradual enhancement of low-energy monopole strength with neutron excess.