Ab initiostudy of shell evolution in neutron-rich Si, S, Ar, and Ca isotopes nearN= 32 and 34

Abstract：

Shell evolution is crucial for understanding nuclear structures across the nuclear chart. In this work, we employed the ab initiovalence space in-medium similarity renormalization group with chiral nucleon-nucleon and three-nucleon interactions to study neutron-rich Si, S, Ar, and Ca isotopes, particularly focusing on nuclei near $ N=32, 34 $ . We systematically analyzed both neutron and proton shell evolutions by examining the excitation energies of the first $ 2^+ $ states and the effective single-particle energies. Our calculations show that the $ N=32 $ sub-shell gradually weakens as protons are removed from the doubly magic nucleus $ ^{52} {\rm{Ca}}$ , eventually disappearing in $ ^{46} {\rm{Si}}$ . Conversely, the strength of the $ N=34 $ sub-shell is enhanced with the removal of protons from $ ^{54} {\rm{Ca}}$ . Furthermore, our results indicate the existence of the proton $ Z=14 $ sub-shell in neutron-rich Si isotopes. These findings suggest that $ ^{48} {\rm{Si}}$ is a doubly magic nucleus, with the excitation energy of the first $ 2^+ $ state around 2.49 MeV, which is approximately 400 keV higher than that of $ ^{54} {\rm{Ca}}$ . This value is comparable to those of other well-known exotic doubly magic nuclei, such as $ ^{52} {\rm{Ca}}$ and $ ^{78} {\rm{Ni}}$ , which is of great interest for further experiments at RIB facilities. In addition, we predicted the low-lying spectra of neutron-rich Si, S, and Ar isotopes, providing new insights for future experiments.