Weakening of shell gap and shape coexistence in 40Mg, 42Si, and 44S

The suppression of N = 28 shell closure is expected towards proton deficient nuclei. As a consequence of which, coexistence of different nuclear shapes occurs in the even-even isotones of N = 28. We have applied covariant density functional theory based on the relativistic energy density functional DD-PCX to study the ground state and shape coexistence in ⁴⁰Mg, ⁴²Si, ⁴⁴S, and ⁴⁶Ar (even-even N = 28 isotones). The binding energy maps, calculated from unconstrained and constrained calculations, indicate a deformed ground state minimum for ⁴⁰Mg, ⁴²Si, and ⁴⁴S. Different nuclear shapes are found to coexist in these nuclei within small excitation energies. In ⁴⁰Mg and ⁴²Si, rigid shapes with different deformations coexist with excitation energies around 2-3 MeV, while ⁴⁴S is expected not to have any particular shape due to a small value of excitation energy. The quenching of shell gap in these nuclei is related to the inversion of neutron orbits with different values of Ω and quadrupole excitations neutrons across fp orbits. For further investigation of shape coexistence in these nuclei, we have employed the triaxial projected shell model in which the deformation parameters, calculated from covariant density functional theory, are taken in input. The presence of $0_2^{+}$ state in the low-lying energy spectra of these nuclei support existence of shape coexistence in ⁴⁰Mg, ⁴²Si, and ⁴⁴S. Furthermore, a reduced value of electric monopole transition strength, in ⁴⁴S, is utilized as a probe for weak mixing of prolately and oblately deformed shapes.