Interplay between nuclear shell evolution and shape deformation revealed by the magnetic moment of ⁷⁵Cu

Exotic nuclei are characterized by having a number of neutrons (or protons) in excess relative to stable nuclei. Their shell structure, which represents single-particle motion in a nucleus^1,2, may vary due to nuclear force and excess neutrons^3–6, in a phenomenon called shell evolution⁷. This effect could be counterbalanced by collective modes causing deformations of the nuclear surface⁸. Here, we study the interplay between shell evolution and shape deformation by focusing on the magnetic moment of an isomeric state of the neutron-rich nucleus ⁷⁵Cu. We measure the magnetic moment using highly spin-controlled rare-isotope beams and achieve large spin alignment via a two-step reaction scheme⁹ that incorporates an angular-momentum-selecting nucleon removal. By combining our experiments with numerical simulations of many-fermion correlations, we find that the low-lying states in ⁷⁵Cu are, to a large extent, of single-particle nature on top of a correlated ⁷⁴Ni core. We elucidate the crucial role of shell evolution even in the presence of the collective mode, and within the same framework we consider whether and how the double magicity of the ⁷⁸Ni nucleus is restored, which is also of keen interest from the perspective of nucleosynthesis in explosive stellar processes.