Relativistic quasiparticle time blocking approximation. II. Pygmy dipole resonance in neutron-rich nuclei

TL;DR

This paper presents a theoretical study of low-lying dipole strength in neutron-rich nuclei using the relativistic quasiparticle time blocking approximation, revealing distinct giant and pygmy dipole resonances.

Contribution

It extends the relativistic quasiparticle random phase approximation (RQRPA) to include coupling with collective vibrations, enabling detailed analysis of dipole spectra in neutron-rich nuclei.

Findings

Identification of two separate dipole resonances: GDR and PDR.

Demonstration of the effectiveness of RQTBA in modeling low-lying dipole strength.

Distinct transition density behaviors for GDR and PDR.

Abstract

Theoretical studies of low-lying dipole strength in even-even spherical nuclei within the relativistic quasiparticle time blocking approximation (RQTBA) are presented. The RQTBA developed recently as an extension of the self-consistent relativistic quasiparticle random phase approximation (RQRPA) enables one to investigate effects of coupling of two-quasiparticle excitations to collective vibrations within a fully consistent calculation scheme based on covariant energy density functional theory. Dipole spectra of even-even $^{130}$Sn -- $^{140}$Sn and $^{68}$Ni -- $^{78}$Ni isotopes calculated within both RQRPA and RQTBA show two well separated collective structures: the higher-lying giant dipole resonance (GDR) and the lower-lying pygmy dipole resonance (PDR) which can be identified by a different behavior of the transition densities of states in these regions.