Physical Review Letters ( IF 8.385 ) Pub Date :
N. Mărginean et al.

The low-spin structure of the semi-magic ${}^{64}$Ni nucleus has been considerably expanded: combining four experiments, several 0${}^{+}$ and 2${}^{+}$ excited states were identified below 4.5 MeV, and their properties established. The Monte Carlo shell model accounts for the results and unveils an unexpectedly-complex landscape of coexisting shapes: a prolate 0${}^{+}$ excitation is located at a surprisingly high energy (3463 keV), with a collective 2${}^{+}$ state 286 keV above it, the first such observation in Ni isotopes. The evolution in excitation energy of the prolate minimum across the neutron $N=40$ sub-shell gap highlights the impact of the monopole interaction and its variation in strength with $N$.

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