Ab initio nuclear shape coexistence and emergence of island of inversion around $N=20$

TL;DR

This paper uses advanced ab initio methods to study shape coexistence and the breakdown of the N=20 magic number in magnesium isotopes, revealing the emergence of the island of inversion and detailed nuclear state configurations.

Contribution

It extends an ab initio framework with IMSRG and generator coordinate methods to accurately reproduce shape coexistence and track the N=20 island of inversion in magnesium isotopes.

Findings

Reproduces coexistence of deformed states at similar energies

Tracks emergence of N=20 island of inversion via IMSRG evolution

Predicts shape isomer in 33Mg with mixed configurations

Abstract

We extend a nuclear ab initio framework based on chiral two- and three-nucleon interactions to investigate shape coexistence and the degradation of the $N=20$ magic number in both even-even and odd-even magnesium isotopes. The quantum-number projected generator coordinate method, combined with the in-medium similarity renormalization group (IMSRG), is employed to compute their low-lying states. This approach reasonably reproduces the coexistence of weakly and strongly deformed states at comparable energies, and allows us to track the emergence of the $N=20$ island of inversion through the continuous IMSRG evolution of the chiral Hamiltonian. Our results indicate that the ground state of $^{33}$Mg with spin-parity $3/2^-$ is predominantly a strongly deformed configuration with $K^\pi = 3/2^-$, while the lowest $7/2^-$ state is predicted to be a shape isomer, consisting of a mixture of weakly deformed configurations with different $K$ values. The results highlight the essential roles of both dynamical and static collective correlations in reproducing the ordering of nuclear states with distinct shapes.