Ab Initio Quadrupole-Invariant Analysis of Neutron-Rich Deformed Nuclei Around N = 28

We present an ab initio quadrupole-invariant study of neutron-rich nuclei around N=28. Effective Hamiltonians and consistently evolved E2 operators are obtained from chiral NN+3N interactions using the ab initio valence-space in-medium similarity renormalization group, and the resulting E2 matrix elements are used to extract the deformation parameters. Along the even–even N=28 isotonic chain, the β values increase systematically from 48Ca to 40Mg, providing a direct deformation signature of the weakening of the N=28 shell closure. The extracted β and γ values show a prolate–oblate–triaxial evolution of the 01+ states from 40Mg to 42Si and 44S, while the 02+ states remain mainly in the prolate-to-triaxial region. Extending the analysis to the neighboring N=27 and 29 isotones, we find a systematic evolution from strongly prolate Mg isotopes, to oblate-side Si isotopes, and to prolate-to-triaxial S isotopes. These results provide a unified picture of quadrupole deformation, triaxiality, and configuration coexistence around N=28.