Collectivity of nuclei near the exotic doubly magic Ni78 by ab initio calculations

Neutron-rich nuclei are attracting significant attention due to vital roles in the nucleosynthesis processes of the universe. The shell evolution which breaks or creates magic numbers in some instances leads to shape coexistence that frequently serves as a portal to the islands of inversion (IOI). ${}^{74}\mathrm{Zn}$ was suggested as a northern extension of the $N=40$ IOI across $Z=28$, and a new IOI around $N=50$ ($Z=20$–28) had also been predicted. We have performed ab initio calculations for even-even Zn isotopes and $N=50$ ($Z>28$) isotones located in the north of the $N=40$ and 50 IOIs. With a chiral two- plus three-nucleon force, the shell-model valence-space effective Hamiltonian is derived using the many-body perturbation theory named $\widehat{\mathrm{Q}}$-box folded diagrams, and the effective operators of electromagnetic transitions are obtained using similar $\widehat{\mathrm{\Theta }}$-box folded diagrams. The calculations reproduce experimental excitation energies and electric qudrupole transition strengths of excited states of the nuclei, showing a collectivity of ${}^{70\text{−}78}\mathrm{Zn}$ isotopes and a noncollectivity of the $N=50$ ($Z>28$) isotones.