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

International audienceExotic 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$^{,}$2 , may vary due to nuclear force and excess neutrons 3$^{–}$6 , in a phenomenon called shell evolution 7 . This effect could be counterbalanced by collective modes causing deformations of the nuclear surface 8 . Here, we study the interplay between shell evolution and shape deformation by focusing on the magnetic moment of an isomeric state of the neutron-rich nucleus$^{75}$Cu. We measure the magnetic moment using highly spin-controlled rare-isotope beams and achieve large spin alignment via a two-step reaction scheme 9 that incorporates an angular-momentum-selecting nucleon removal. By combining our experiments with numerical simulations of many-fermion correlations, we find that the low-lying states in$^{75}$Cu are, to a large extent, of single-particle nature on top of a correlated$^{74}$Ni core. We elucidate the crucial role of shell evolution even in the presence of the collective mode, and within the same framework we consider whether and how the double magicity of the$^{78}$Ni nucleus is restored, which is also of keen interest from the perspective of nucleosynthesis in explosive stellar processes