Ab initio mapping of the boundary of the N = 20 island of inversion

Starting from a chiral two- plus three-nucleon interaction, we perform a systematic study of the low-lying states of neutron-rich nuclei around $N=20$ using the in-medium generator coordinate method (IM-GCM), which combines the multi-reference in-medium similarity renormalization group (MR-IMSRG) with the quantum-number projected generator coordinate method (PGCM) defined in a full single-particle space. The main features of the energy spectra and electromagnetic properties of low-lying states in both even-even and odd-mass nuclei of this mass region are reasonably reproduced. The boundary of the $N=20$ island of inversion (IOI) is investigated, and the results indicate that ³⁰Ne, 29,31,33Na, ³¹⁻³⁴Mg, and ³⁵Al lie within the IOI, whereas ²⁹F, ²⁹Ne, ³⁰Mg, 31, 33Al, 34,35Si, and ³⁵P fall outside it.