Low-energy monopole strength in exotic Nickel isotopes

TL;DR

This paper predicts low-energy monopole states in neutron-rich Nickel isotopes using microscopic models, revealing almost pure neutron excitations that become more prominent with increasing neutron number.

Contribution

It demonstrates the existence of pronounced low-energy monopole states in exotic Ni isotopes using both Skyrme HF+RPA and relativistic RHB+RQRPA models, highlighting their neutron single-hole excitations.

Findings

Pronounced monopole states between 10-15 MeV in neutron-rich Ni isotopes.

States are almost pure neutron single-hole excitations.

Strength of these states increases with neutron excess.

Abstract

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 MeV and 15 MeV, well separated from the isoscalar GMR. 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 (Q)RPA strength is considerably enhanced by the residual interaction. The theoretical analysis predicts the gradual enhancement of low-energy monopole strength with neutron excess.