Low-lying states of neutron-rich $N=50$ isotones in multi-reference covariant density functional theory

Abstract

Predictions for the systematics of low-lying states in neutron-rich $N=50$ isotones from \nuclide[80]{Zn} to \nuclide[70]{Ca}, where experimental information remains scarce, exhibit substantial model dependence. We investigate these nuclei using multi-reference covariant density functional theory based on a relativistic energy density functional, in which nuclear wave functions are constructed as superpositions of symmetry-restored mean-field states with different quadrupole deformations. The low-lying states of \nuclide[78]{Ni} are reasonably reproduced, and a prolate rotational band built on the second $0^+$ state is predicted in addition to a near-spherical ground state. As the proton number decreases toward $Z=20$, the low-energy structure exhibits a nonmonotonic evolution from spherical to strongly deformed and subsequently back to spherical shapes, consistent with valence-space shell-model predictions. In particular, the emergence of deformed ground states in \nuclide[76]{Fe} and \nuclide[74]{Cr} is accompanied by enhanced quadrupole collectivity and stronger shape mixing. The results provide a coherent microscopic description of the structural evolution from the doubly magic \nuclide[78]{Ni} to the neutron-rich $N=50$ isotones.