Relativistic quasiparticle random-phase approximation description of isoscalar compression modes in open-shell nuclei in the A≅60 mass region

Very recent inelastic α-scattering data on the isoscalar monopole and dipole strength distributions in 56Fe, 58Ni, and 60Ni are analyzed in the relativistic quasiparticle random-phase approximation (RQRPA) with the DD-ME2 effective nuclear interaction (nuclear matter compression modulus Knm=251 MeV). The calculation nicely reproduces the observed asymmetric shapes of the monopole strength, and the bimodal structure of the dipole strength distributions. The RQRPA centroid and mean energies are in very good qualitative agreement with the experimental values both for the monopole, and for the low- and high-energy components of the dipole transition strengths. It is noted, however, that while DD-ME2 reproduces in detail the excitation energies of the giant monopole resonances (GMR) in nuclei with A≥90, the theoretical centroids are systematically above the experimental values in lighter nuclei with A≤60. The latter can be reproduced with an effective interaction with a lower value of Knm≅230 MeV but, because of the asymmetric shapes and pronounced fragmentation of the monopole strength distributions, isoscalar GMR data in light nuclei cannot provide accurate estimates of the nuclear matter compression modulus