Individual low-energy toroidal dipole state in $^{24}$Mg

TL;DR

This paper predicts the existence of a low-energy toroidal dipole state in $^{24}$Mg using QRPA calculations, highlighting its unique vortical nature and potential for experimental detection in highly deformed nuclei.

Contribution

It identifies and characterizes an individual low-energy toroidal dipole state in $^{24}$Mg, demonstrating its vortical structure and robustness across different Skyrme interactions.

Findings

The lowest $1^-1$ excitation in $^{24}$Mg is a vortical toroidal state.

This state is easier to discriminate experimentally than the toroidal dipole resonance.

The toroidal state is linked to the nucleus's prolate deformation.

Abstract

The low-energy dipole excitations in $^{24}$Mg are investigated within the Skyrme quasiparticle random-phase-approximation (QRPA) for axial nuclei. The calculations with the force SLy6 reveal a remarkable feature: the lowest $I^{\pi}K=1^-1$ excitation (E = 7.92 MeV) in $^{24}$Mg is a vortical toroidal state (TS) representing a specific vortex-antivortex realization of well-known spherical Hill's vortex in a strongly deformed axial confinement. This is a striking example of an {\it individual} TS which can be much easier discriminated in experiment than the toroidal dipole resonance embracing many states. The TS acquires the lowest energy due to the huge prolate axial deformation in $^{24}$Mg. The result persists for different Skyrme parameterizations (SLy6, SVbas, SkM*). We analyze spectroscopic properties of the TS and its relation with the cluster structure of $^{24}$Mg. Similar TS could exist in other highly prolate light nuclei. They could serve as promising tests for various reactions to probe a vortical (toroidal) nuclear flow.