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

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 nucleus1,2, may vary due to nuclear forceand excess neutrons3–6, 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 nucleosynthesisin explosive stellar processes.