Erosion of N = 28 shell closure: Shape coexistence and monopole transition

In neutron-rich nuclei neighboring 42Si, the quenching of the N = 28 shell gap occurs and is expected to induce the shape coexistence in their excitation spectra. We have applied the theoretical framework of antisymmetrized molecular dynamics with Gogny D1S density functional to describe the shape coexistence in N = 28 isotones 40Mg, 42Si, and 44S. We show that different nuclear shapes coexist in these nuclei: The rigid shapes with different deformations coexist in 40Mg and 42Si, while 44S exhibits large-amplitude collective motion and does not have any particular shape. These characteristics are reflected well in the monopole transition strengths which can be utilized as a probe for the shape coexistence.