Shape Changes in the N = 28 Island of Inversion: Collective Structures Built on Configuration-Coexisting States in S 43

The neutron-rich nuclei in the $N=28$ island of inversion have attracted considerable experimental and theoretical attention, providing great insight into the evolution of shell structure and nuclear shape in exotic nuclei. In this work, for the first time, quadrupole collectivity is assessed simultaneously on top of the $3/{2}^{\ensuremath{-}}$ ground state and the $7/{2}^{\ensuremath{-}}$ shape-coexisting isomer of $^{43}\mathrm{S}$, putting the unique interpretation of shape and configuration coexistence at $N=27$ and 28 in the sulfur isotopic chain to the test. From an analysis of the electromagnetic transition strengths and quadrupole moments predicted within the shell model, it is shown that the onset of shape coexistence and the emergence of a simple collective structure appear suddenly in $^{43}\mathrm{S}$ with no indication of such patterns in the $N=27$ isotone $^{45}\mathrm{Ar}$.