Ab initio nuclear shape coexistence and emergence of island of inversion around N = 20

We extend a nuclear ab initio framework based on chiral two- and three-nucleon interactions to investigate shape coexistence and the degradation of the N = 20 magic number in both even-even and odd-even magnesium isotopes. The quantum-number projected generator coordinate method, combined with the in-medium similarity renormalization group (IMSRG), is employed to compute their low-lying states. This approach reasonably reproduces the coexistence of weakly and strongly deformed states at comparable energies, and allows us to track the emergence of the N = 20 island of inversion through the continuous IMSRG evolution of the chiral Hamiltonian. Our results indicate that the ground state of 33 Mg with spin-parity 3 / 2 − is predominantly a strongly deformed configuration with K π = 3 / 2 − , while the lowest 7 / 2 − state is predicted to be a shape isomer, consisting of a mixture of weakly deformed configurations with different K values. The results highlight the essential roles of both dynamical and static collective correlations in reproducing the ordering of nuclear states with distinct shapes.