Low-energy monopole strength in spherical and axially deformed nuclei: Cluster and soft modes

Background: Several recent experiments report significant low-energy isoscalar monopole strength, below the giant resonance, in various nuclei. In light alpha-conjugate nuclei, these low-energy resonances were recently interpreted as cluster vibration modes. However, the nature of these excitations in neutron-rich nuclei remains elusive. Purpose: The present work provides a systematic analysis of the low-energy monopole strength in isotopic chains, from neon to germanium, in order to monitor and understand its nature and conditions of emergence. Methods: We perform covariant quasiparticle random phase approximation calculations, formulated within the finite amplitude method, on top of constrained relativistic Hartree-Bogoliubov (RHB) reference states. Results: Neutron excess leads to the appearance of low-energy excitations according to a systematic pattern reflecting the single-particle features of the underlying RHB reference state. With the onset of deformation, these low-energy resonances get split and give rise to more complex patterns, with possible mixing with the giant resonance. At lower energy, clusterlike excitations found in N = Z systems survive in neutron-rich nuclei, with valence neutrons arranging in molecularlike orbitals. Finally, at very low energy, pair excitations are also found in superfluid nuclei, but remain negligible in most of the cases. Conclusions: The low-energy part of the monopole strength exhibits various modes, from cluster vibrations (approximate to 5-10 MeV) to components of the giant resonance downshifted by the onset of deformation, including soft modes (approximate to 10-15 MeV) as well as pair excitation (<5 MeV), with possible mixing, depending on neutron excess, deformation, and pairing energy.

Automated summary

This paper presents a systematic analysis of the low-energy monopole strength in different isotopic chains. The results show that the nature of these excitations varies with neutron excess and deformation, resulting in different modes such as cluster vibrations, downshifted components of the giant resonance, and pair excitations.