Low-lying dipole resonance in neutron-rich Ne isotopes

Abstract

Microscopic structure of the low-lying isovector dipole excitation mode in neutron-rich $^{26,28,30}\mathrm{Ne}$ is investigated by performing deformed quasiparticle-random-phase-approximation (QRPA) calculations. The particle-hole residual interaction is derived from a Skyrme force through a Landau-Migdal approximation. We obtain the low-lying resonance in $^{26}\mathrm{Ne}$ at around 8.6 MeV. It is found that the isovector dipole strength at ${E}_{x}<10$ MeV exhausts about 6.0% of the classical Thomas-Reiche-Kuhn dipole sum rule. This excitation mode is composed of several QRPA eigenmodes, one is generated by a $\ensuremath{\nu}(2{s}_{1/2}^{\ensuremath{-}1}2{p}_{3/2})$ transition dominantly and the other mostly by a $\ensuremath{\nu}(2{s}_{1/2}^{\ensuremath{-}1}2{p}_{1/2})$ transition. The neutron excitations take place outside of the nuclear surface reflecting the spatially extended structure of the $2{s}_{1/2}$ wave function. In $^{30}\mathrm{Ne}$, the deformation splitting of the giant resonance is large, and the low-lying resonance overlaps with the giant resonance.