Charge-exchange dipole excitations in deformed nuclei

TL;DR

This study investigates how nuclear deformation affects charge-exchange dipole excitations, revealing deformation splitting and the potential emergence of pygmy dipole resonances in neutron-rich nuclei using advanced energy-density functional methods.

Contribution

It provides a detailed analysis of deformation effects on charge-exchange dipole modes, including the splitting into K-components and the possible appearance of pygmy resonances in deformed neutron-rich nuclei.

Findings

Deformation causes splitting into K=0 and K=±1 components in charge-exchange dipole excitations.

The splitting magnitude is proportional to nuclear deformation.

Pygmy dipole resonances may appear in deformed neutron-rich nuclei.

Abstract

Background: The electric giant-dipole resonance (GDR) is the most established collective vibrational mode of excitation. A charge-exchange analog, however, has been poorly studied in comparison with the spin (magnetic) dipole resonance (SDR). Purpose: I investigate the role of deformation on the charge-exchange dipole excitations and explore the generic features as an isovector mode of excitation. Methods: The nuclear energy-density functional method is employed for calculating the response functions based on the Skyrme--Kohn--Sham--Bogoliubov method and the proton-neuton quasiparticle-random-phase approximation. Results: The deformation splitting into $K=0$ and $K=\pm 1$ components occurs in the charge-changing channels and is proportional to the magnitude of deformation as is well known for the GDR. For the SDR, however, a simple assertion based on geometry of a nucleus cannot be applied for explaining the vibrational frequencies of each $K$-component. A qualitative argument on the strength distributions for each component is given based on the non-energy-weighted sum rules taking nuclear deformation into account. The concentration of the electric dipole strengths in low energy and below the giant resonance is found in neutron-rich unstable nuclei. Conclusions: The deformation splitting occurs generically for the charge-exchange dipole excitions as in the neutral channel. The analog pygmy dipole resonance can emerge in deformed neutron-rich nuclei as well as in spherical systems.