Pygmy dipole mode in deformed neutron-rich Mg isotopes close to the drip line

TL;DR

This paper studies low-lying dipole excitations in neutron-rich Mg isotopes near the drip line, revealing collective neutron skin oscillations and deformation effects using advanced microscopic models.

Contribution

It introduces a detailed microscopic analysis of dipole modes in deformed neutron-rich Mg isotopes near the drip line, highlighting the development of low-energy collective excitations and mode couplings.

Findings

Low-lying dipole strength increases near the drip line.

Neutron skin oscillates against the isoscalar core.

Deformation enhances coupling between dipole and octupole modes.

Abstract

We investigate the microscopic structure of the low-lying isovector-dipole excitation mode in neutron-rich $^{36,38,40}$Mg close to the drip line by means of the deformed quasiparticle-random-phase-approximation employing the Skyrme and the local pairing energy-density functionals. It is found that the low-lying bump structure above the neutron emission-threshold energy develops when the drip line is approached, and that the isovector dipole strength at $E_{x} < 10$ MeV exhausts about $6.0 %$ of the classical Thomas-Reiche-Kuhn dipole sum rule in $^{40}$Mg. We obtained the collective dipole modes at around 8-10 MeV in Mg isotopes, which consist of many two-quasiparticle excitations of neutron. The transition density clearly shows an oscillation of the neutron skin against the isoscalar core. We found significant coupling effects between the dipole and octupole excitation modes due to the nuclear deformation. It is also found that the responses for the compressional dipole and isoscalar octupole excitations are much enhanced in the lower energy region.