Quenching of N = 28 shell gap and a low-lying quadrupole mode in the vicinity of neutron-rich N = 28 isotones

TL;DR

This paper explores how the quenching of the N=28 shell gap in neutron-rich isotones leads to low-energy quadrupole modes, highlighting the role of pairing and proton-neutron correlations in these nuclear excitations.

Contribution

It introduces a self-consistent theoretical approach to explain low-lying quadrupole states in N=28 isotones, emphasizing the impact of shell gap quenching and correlations.

Findings

Emergence of low-energy quadrupole modes in certain isotones.

Significant role of pairing and proton-neutron correlations.

Calculated B(E2) values align with experimental data.

Abstract

We investigate the low-lying 2$^+$ states of $N=28$ isotones ($^{48}$Ca, $^{46}$Ar, $^{44}$S and $^{42}$Si) with using the canonical-basis time-dependent Hartree-Fock-Bogoliubov theory in which the pairing is taken into account self-consistently. The quadrupole mode with very small excitation energies emerges in $^{46}$Ar, $^{44}$S and $^{42}$Si due to the strong quadrupole correlations triggered by the quenching of the $N=28$ shell gap. The importance of the quadrupole correlations between the protons and neutrons, and the role of the pairing correlation to reinforce the lowest 2$^+$ mode are discussed. It is also shown that the $B$(E2$\uparrow$) values of $^{48}$Ca and $^{46}$Ar are plausibly explained by our calculations.