Octupole deformation in even-even Ra isotopes from covariant density functional theory with localized exchange terms in a three-dimensional lattice space

Abstract

The covariant density functional theory in a three-dimensional lattice space is extended to the PCF-PK1 functional with localized exchange terms and is employed to study the nuclear shape evolution of even-even Ra isotopes. Well-developed axial octupole deformations are found for the ground states of $^{222-228}$Ra with no evidence of triaxial shapes. The energy gain of octupole deformation is employed to assess the stability of octupole deformation, with relatively larger values observed for $^{224}$Ra and $^{226}$Ra. A simplified analysis method based on the single-particle spectrum at the octupole deformation parameter $\beta_3=0$ is proposed to identify the key single-particle levels driving octupole deformation. It is found that the $m_z=3/2$ orbitals from $\nu 1j_{15/2}$ and $\nu 2g_{9/2}$ and the $m_z=1/2$ orbitals from $\pi 1i_{13/2}$ and $\pi 2f_{7/2}$, play crucial roles in the formation of octupole deformation in Ra isotopes. Furthermore, increasing the tensor coupling strength promotes octupole deformation, whereas reducing the pairing strength stabilizes it. Our results provide a microscopic understanding of octupole deformation in the Ra isotopic chain and highlight the importance of both tensor coupling and pairing correlations in reflection-asymmetric nuclear shapes.