Low-energy isovector and isoscalar dipole response in neutron-rich nuclei

TL;DR

This study uses relativistic RPA to analyze low-energy dipole responses in neutron-rich nuclei, revealing the emergence and characteristics of pygmy dipole states influenced by symmetry energy parameters.

Contribution

It predicts the presence and properties of pygmy dipole states in various nuclei using a relativistic framework, linking their behavior to symmetry energy parameters.

Findings

Pygmy dipole states are predicted in both isovector and isoscalar channels.

The strength of PDS increases with the symmetry energy at saturation and its slope.

PDS exhausts a small fraction of the energy-weighted sum rule but a larger part of the inverse energy-weighted sum rule.

Abstract

The self-consistent random phase approximation (RPA), based on the framework of relativistic energy density functionals, is employed in the study of isovector and isoscalar dipole response in $^{68}$Ni, $^{132}$Sn, and $^{208}$Pb. The evolution of pygmy dipole states (PDS) in the region of low excitation energies is analyzed as a function of the density-dependence of the symmetry energy for a set of relativistic effective interactions. The occurrence of PDS is predicted in the response to both the isovector and isoscalar dipole operators, and its strength is enhanced with the increase of the symmetry energy at saturation and the slope of the symmetry energy. In both channels the PDS exhausts a relatively small fraction of the energy-weighted sum rule but a much larger percentage of the inverse energy-weighted sum rule. For the isovector dipole operator the reduced transition probability $B(E1)$ of the PDS is generally small because of pronounced cancellation of neutron and proton partial contributions. The isoscalar reduced transition amplitude is predominantly determined by neutron particle-hole configurations, most of which add coherently, and this results in a collective response of the PDS to the isoscalar dipole operator.