Tensor-force effects on single-particle levels and proton bubble structure around the $Z$ or $N=20$ magic number

Abstract

Applying the semi-realistic $NN$ interactions that include realistic tensor force to the Hartree-Fock calculations, we investigate tensor-force effects on the single-particle levels in the Ca isotopes. The semi-realistic interaction successfully describes the experimental difference between $\varepsilon(p1s_{1/2})$ and $\varepsilon(p0d_{3/2})$ (denoted by $\mathit{\Delta}\varepsilon_{13}$) both at $^{40}$Ca and $^{48}$Ca, confirming importance of the tensor force. The tensor force plays a role in the $N$-dependence of $\mathit{\Delta}\varepsilon_{13}$ also in neutron-rich Ca nuclei. While the $p1s_{1/2}$-$p0d_{3/2}$ inversion is predicted in heavier Ca nuclei as in $^{48}$Ca, it takes place only in $N\geq 46$, delayed by the tensor force. We further investigate possibility of proton bubble structure in Ar, which is suggested by the $p1s_{1/2}$-$p0d_{3/2}$ inversion in $^{48}$Ca and more neutron-rich Ca nuclei, by the spherical Hartree-Fock-Bogolyubov calculations. Even with the inversion at $^{48}$Ca the pair correlation prohibits prominent bubble distribution in $^{46}$Ar. Bubble in Ar is unlikely also near neutron drip line because either of unboundness or of deformation. However, $^{34}$Si remains a candidate for proton bubble structure, owing to large shell gap between $p1s_{1/2}$ and $p0d_{5/2}$.