Charge Radii of Ni-55;56 Reveal a Surprisingly Similar Behavior at N=28 in Ca and Ni Isotopes

Nuclear charge radii of Ni-55;56 were measured by collinear laser spectroscopy. The obtained information completes the behavior of the charge radii at the shell closure of the doubly magic nucleus Ni-56. The trend of charge radii across the shell closures in calcium and nickel is surprisingly similar despite the fact that the Ni-56 core is supposed to be much softer than the Ca-48 core. The very low magnetic moment mu(Ni-55) = -1.108(20) mu(N) indicates the impact of M1 excitations between spin-orbit partners across the N; Z = 28 shell gaps. Our charge-radii results are compared to ab initio and nuclear density functional theory calculations, showing good agreement within theoretical uncertainties.