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

TL;DR

This study uses nuclear density-functional theory to analyze the gamma vibrational mode in neutron-rich Dy isotopes, explaining the observed energy softening at N=106 and the enhanced collectivity at N=108-110.

Contribution

It provides a microscopic explanation for the isotopic dependence of gamma vibration energy in neutron-rich rare-earth nuclei using Skyrme-EDF QRPA calculations.

Findings

Reproduces the lowering of gamma vibrational energy around N=106.

Identifies the role of specific neutron orbitals in gamma collectivity.

Predicts strong collectivity at N=108-110 due to Fermi level positioning.

Abstract

Background: The $\gamma$ vibrational mode of excitation is an acknowledged collective mode in deformed nuclei. The collectivity depends on the details of the shell structure around the Fermi levels, in particular the presence of the orbitals that have the enhanced transition matrix elements of the non-axial quadrupole excitation. Quite recently, a sudden decrease in the excitation energy of the $\gamma$ vibration was observed at RIKEN RIBF for the neutron-rich Dy isotopes at $N=106$. Purpose: In the present work, by studying systematically the microscopic structure of the $\gamma$ vibration in the neutron-rich Dy isotopes with $N=98-114$, we try to understand the mechanism of the observed softening. Methods: 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. Results: The lowering of the excitation energy around $N=106$ is reproduced well by employing the SkM* and SLy4 functionals. We find the similar isotopic dependence of the excitation energy in the neutron-rich Er and Yb isotopes as well. Conclusions: The microscopic framework of the Skyrme-EDF based QRPA describes well the isotopic dependence of the energy of the $\gamma$ vibration in the well-deformed neutron-rich rare-earth nuclei. 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 $N=106$.