Enhanced quadrupole collectivity in doubly-magic 56Ni: Lifetime measurements of the 4 1 + and 6 1 + states

Lifetime measurements of excited states in doubly-magic ⁵⁶Ni have been performed exploiting the Doppler-shift attenuation method in order to determine reduced transition probabilities. For the $4_1^{+}$ and $6_1^{+}$ states, the deduced $B\left(\mathrm{E2}\right)$ values are compared with results from shell-model calculations employing the GXPF1A and the modern PFSDG-U interactions. In addition, valence ab-initio calculations were performed using a novel realistic Hamiltonian derived from chiral perturbation theory including three-body potential contributions and are confronted with the experimental findings. The new results show maximum E2 strength in comparison with known values along the $N=28$ chain of isotones. The results corroborate the high collectivity for the double shell closure at $N=Z=28$ which was anticipated from the large $B(\mathrm{E2};2_1^{+}\to 0_{g.s.}^{+})$ value despite the considerable increase of its excitation energy as compared to neighboring semi-magic nuclei. Based on similarities in the shell structures of the self-conjugate doubly-magic nuclei ⁵⁶Ni and ¹⁰⁰Sn, the new values could be an indication for an expected comparable collective behavior of the $6_1^{+}$ state in ¹⁰⁰Sn.