Energy-dependence of skin-mode fraction in $E1$ excitations of neutron-rich nuclei

TL;DR

This study analyzes the energy-dependent behavior of skin-mode and proton-neutron mode contributions in low-energy E1 excitations of neutron-rich nuclei, revealing a universal ratio pattern across different nuclei.

Contribution

It demonstrates a generic, energy-dependent ratio of skin-mode strength to total E1 strength, largely independent of specific nuclei or effective interactions.

Findings

Confirmed crossover from skin mode to pn mode with increasing energy.

Identified the ratio of skin-mode strength as a universal function of excitation energy.

Showed insensitivity of this ratio to different nuclei and effective interactions.

Abstract

We have extensively investigated characters of the low-energy $E1$ strengths in $N>Z$ nuclei, by analyzing the transition densities obtained by the HF+RPA calculations with several effective interactions. Crossover behavior has been confirmed, from the skin mode at low energy to the $pn$ mode at higher energy. Decomposing the $E1$ strengths into the skin-mode, $pn$-mode and interference fractions, we show that the ratio of the skin-mode strength to the full strength may be regarded as a generic function of the excitation energy, insensitive to nuclides and effective interactions, particularly beyond Ni.