Crossover from skin-mode to proton-neutron-mode in $E1$ excitations of neutron-rich nuclei

TL;DR

This paper investigates the nature of low-energy E1 excitations in neutron-rich nuclei, revealing a crossover from skin-mode to proton-neutron mode with increasing energy, using a novel decomposition method based on transition densities.

Contribution

It introduces a new decomposition method to distinguish skin and proton-neutron modes in E1 excitations and demonstrates the crossover behavior in doubly-magic nuclei.

Findings

Crossover from skin mode to pn mode observed with increasing energy.

The skin-mode strength ratio is consistent across different nuclei and interactions.

Low-energy E1 excitations are not always dominated by skin mode, exemplified in 90Zr.

Abstract

Character of the low-energy $E1$ excitations is investigated by analyzing transition densities obtained from the RPA calculations in the doubly-magic nuclei. We propose a decomposition method of the $E1$ excitations into the $pn$ mode (i.e. oscillation between protons and neutrons) and the skin mode (oscillation of the neutron skin against the inner core) via the transition densities, by which their mixing is handled in a straightforward manner. Crossover behavior of the $E1$ excitations is found, from the skin mode at low energy to the $pn$ mode at higher energy. The ratio of the skin-mode strength to the full strength turns out to be insensitive to nuclide and to effective interactions in the energy region of the crossover. Depending on the excitation energy, the observed low-energy $E1$ excitations are not necessarily dominated by the skin mode, as exemplified for $^{90}$Zr. fied for $^{90}$Zr.