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

• Received 18 November 2017
• Revised 11 January 2018

DOI:https://doi.org/10.1103/PhysRevLett.120.182501