Low-lying 2$^+$ states generated by $pn$-quadrupole correlation and $N=28$ shell quenching

Abstract

The quadrupole vibrational modes of neutron-rich N =28 isotones ( 48 Ca, 46 Ar, 44 S and 42 Si) are investigated using the canonical-basis time-dependent Hartree-Fock-Bogoliubov theory with several choice of energy density functionals, including nuclear pairing correlation. It is found that the quenching of N =28 shell gap and the proton holes in the sd-shell trigger quadrupole correlation and increase the collectivity of the low-lying 2 + state in 46 Ar. It is also found that the pairing correlation plays an important role to increase the collectivity. We also demonstrate that the same mechanism to enhance the low-lying collectivity applies to other N =28 isotones 44 S and 42 Si, and it generates a couple of low-lying 2 + states which can be associated with the observed 2 + states.