Triple Shape Coexistence in Stable $^{64}$Ni

In a tour de force involving 4 experiments at 4 different laboratories, an unexpectedly-complex landscape of triple shape coexistence was established in the stable, semi-magic $^{64}$Ni nucleus. For the first time, an excited $2^+$ state was also found in the prolate minimum. Shell model calculations, performed with significantly extended basis vector space, provided a microscopic understanding of the results and of the evolution in excitation energy of the prolate minimum across the $N=40$ sub-shell gap, and highlighted the impact of the monopole interaction and its variation in strength with increasing $N$.