The shapes of neutron-rich exotic Ni isotopes are studied. Large-scale shell model calculations are performed by the advanced Monte Carlo shell model (MCSM) for the $pf$-$g_{9/2}$-$d_{5/2}$ model space. Experimental energy levels are reproduced well by a single fixed Hamiltonian. Intrinsic shapes are analyzed for MCSM eigenstates. Intriguing interplays among spherical, oblate, prolate, and $\gamma$-unstable shapes are seen, including shape fluctuations, E(5)-like situations, the magicity of doubly magic ${}^{56,68,78}$Ni, and the coexistence of spherical and strongly deformed shapes. Regarding the last point, strong deformation and change of shell structure can take place simultaneously, being driven by the combination of the tensor force and changes of major configurations within the same nucleus.

• Received 19 September 2013
• Revised 25 November 2013

DOI:https://doi.org/10.1103/PhysRevC.89.031301