Low-energy dipole excitation mode in $^{18}$O with antisymmetrized molecular dynamics

TL;DR

This study investigates low-energy dipole excitations in $^{18}$O using antisymmetrized molecular dynamics, revealing two states with mixed toroidal and compressive dipole modes and distinct structural features.

Contribution

It introduces a novel analysis of LED states in $^{18}$O, demonstrating the coexistence and mixing of TD and CD modes in a light nucleus using advanced computational methods.

Findings

Identified two LED states with different structural characteristics.

Both states exhibit significant mixed TD and CD strengths.

Contrasts LED mode behavior in $^{18}$O with that in deformed nuclei.

Abstract

Low-energy dipole (LED) excitations in $^{18}$O were investigated using a combination of the variation after $K$-projection in the framework of antisymmetrized molecular dynamics with $\beta$-constraint with the generator coordinate method. We obtained two LED states, namely, the $1_1^-$ state with a dominant shell-model structure and the $1_2^-$ state with a large $^{14}\textrm{C}+\alpha$ cluster component. Both these states had significant toroidal dipole (TD) and compressive dipole (CD) strengths, indicating that the TD and CD modes are not separated but mixed in the LED excitations of $^{18}$O. This is unlike the CD and TD modes for well-deformed nuclei such as $^{10}$Be, where the CD and TD modes are generated as $K^{\pi}=0^-$ and $K^{\pi}=1^-$ excitations, respectively, in a largely deformed state.