Shape Coexistence in Neutron-Rich Nickel Isotopes around $N =$ 40

Shape coexistence is a fascinating phenomenon in atomic nuclei characterized by multiple states with different intrinsic shapes coexisting at similar excitation energies. In even-even nuclei, a hallmark of shape coexistence is low-energy 0$^{+}$ states. In $^{68}$Ni, the Monte-Carlo Shell Model (MCSM) employing the A3DA interaction, utilizing the \textit{fpg}$_{9/2}d_{5/2} $model space for protons and neutrons, predicts triple shape coexistence with three 0$^{+}$ states below 3 MeV. Transitioning to $^{70}$Ni, the energy of the prolate-deformed 0$^{+}$ state is predicted to drop precipitously from 2511 to 1525 keV. This is due to strengthening of the attractive $\nu g_{9/2}-\pi f_{5/2}$ and repulsive $\nu g_{9/2}-\pi f_{7/2}$ monopole interactions of the tensor force altering the effective single-particle energies of the $\pi f_{7/2}$ and $\pi f_{5/2}$ single-particle states, thereby reducing the spherical $Z =$ 28 shell gap. Recent beta-decay spectroscopy experiments at the National Superconducting Cyclotron Laboratory (NSCL) have discovered a new excited 0$^{+}$ state at 1567 keV in $^{70}$Ni. This result supports MCSM predictions extending the picture of shape coexistence to $^{70}$Ni and demonstrates the importance of the tensor force for describing the nuclear structure of neutron-rich nuclei. Results of the latest NSCL experiments will be presented.