  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.
