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.

Comments: 7 page and 4 figures