  Background: 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. Purpose: We show that different nuclear shapes coexist in N
= 28 isotones 40Mg, 42Si, and 44S, and investigate observables to probe it.
Method: Antisymmetrized molecular dynamics with Gogny D1S density functional is
applied to describe the shape coexistence phenomena without ad hoc assumption
of the nuclear shape. Results: We find that rigid shapes with different
deformations coexist in the ground and the first excited 0+ states of 40Mg and
42Si, while in 44S the states exhibit large-amplitude collective motion, which
does not have any particular shape. These characteristics are reflected well in
the monopole transition strengths. Conclusion: The quenching of the N = 28
shell gap leads to the unique shape coexistence in the N = 28 isotones, which
can be probed by the monopole transition strengths.
