The region of neutron-rich nuclei surrounding $N=28$ has garnered attention due to experimental and theoretical evidence indicating a diversity of shape and configuration coexistence. In the present Letter, we have performed a comprehensive study for these nuclei utilizing ab initio valence-space in-medium similarity renormalization group calculations with chiral nucleon-nucleon and three-nucleon interactions. First, the band structures of ${}^{41,43}\mathrm{S}$ and ${}^{45}\mathrm{Ar}$ are established via calculated spectra and $E2$ maps. The results well reproduce the $K=1/2$ ground state band and the $K=7/2$ band built upon the $7/2_1^{-}$ isomer in ${}^{43}\mathrm{S}$, which coexists with its $3/2_1^{-}$ ground state. Subsequently, the band structures of neutron-rich $N=27$ and $N=29$ isotones are discussed, with a focus on the potential emergence of configuration coexistence and isomers. We predict the presence of a similar configuration-coexisting $7/2_1^{-}$ isomeric state in ${}^{45}\mathrm{S}$, as an extension from ${}^{43}\mathrm{S}$. Furthermore, our calculations predict that the configuration-coexisting states also occur in the unbound ${}^{39}\mathrm{Mg}$ nucleus. Our ab initio calculations provide crucial insights into the collective structures of neutron-rich nuclei around $N=28$, specifically elucidating the phenomenon of configuration coexistence and isomers.

• Received 24 November 2023
• Revised 31 January 2024
• Accepted 18 March 2024

DOI:https://doi.org/10.1103/PhysRevC.109.L041301