Enhanced collectivity of γ vibration in neutron-rich Dy isotopes with N = 108–110

Abstract

The γ vibrational mode of excitation is an acknowledged collective mode in deformed nuclei. Quite recently, a sudden decrease in the excitation energy of the γ vibration was observed at RIKEN for the neutron-rich Dy isotopes at N = 106. In the present work, by studying systematically the microscopic structure of the γ vibration in the neutron-rich Dy isotopes with N = 98–114, we try to understand the mechanism of the observed softening. The low-frequency modes of excitation in the neutron-rich rare-earth nuclei are described based on nuclear density functional theory. We employ the Skyrme energy density functionals (EDF) in the Hartree–Fock–Bogoliubov calculation for the ground states and in the quasiparticle random phase approximation (QRPA) for the excitations. The lowering of the excitation energy around N = 106 is reproduced well by employing the SkM* and SLy4 functionals. It is found that the coherent contribution of the ν[512]3/2 ν[510]1/2, ν[510]1/2 ν[512]5/2, and ν[512]3/2 ν[514]7/2 excitations satisfying the selection rule of the non-axial quadrupole matrix element plays a major role in generating the collectivity. We find a similar isotopic dependence of the excitation energy in the neutron-rich Er and Yb isotopes. The strong collectivity at N = 108–110 is expected as the Fermi level of neutrons lies just among the orbitals that play an important role in generating the collectivity around N = 106.