Monopole modes of excitation in deformed neutron-rich Mg isotopes

Collective motion in unstable nuclei has raised a considerable interest both experimentally and theoretically. This is because low-frequency modes of excitation are quite sensitive to the shell structure near the Fermi level, and we can expect unique excitation modes to emerge associated with the new spatial structures such as neutron skins and the novel shell structures that generate new regions of deformation. In order to investigate new kinds of collective excitation modes in exotic nuclei, the random phase approximation (RPA) based on the self consistent mean field has been employed by many groups. They are however largely restricted to spherical systems, and the low-frequency excitation modes in deformed neutron-rich nuclei remain mostly unexplored. Quite recently, we have developed a new framework of the deformed quasiparticle-RPA (QRPA) on top of the Skyrme-Hartree-Fock-Bogoliubov (SHFB) mean field formulated in the cylindrical coordinates [1]. By employing the Skyrme SkM functional and the mixed-type pairing energy density functional, we solve the QRPA equations, the continuum states being discretized by a box boundary condition. The momentum dependence is explicitly taken into account in the residual interactions. We apply the new calculation scheme to the monopole modes of excitation in neutron-rich Magnesium isotopes with N = 22, 24, 26 and 28 close to the drip line. The neutron-rich Mg isotopes under investigation are found to have prolately deformed ground states. Figure 1 shows the isoscalar monopole transition strengths. We obtain two resonance peaks at around 12 MeV and 25 MeV. The higher-frequency peak corresponds to the typical giant-monopole resonance appearing at 2hω. The lower-frequency peak corresponds to the Kπ = 0+ component of the giant quadrupole resonance. Besides the giant resonances, we can see a prominent peak at about 3 MeV, below the neutron emission threshold. The low-lying mode is mainly generated by the superposition of two-quasineutron excitations around the Fermi level, whose wave functions have spatially extended structure.(author)