Influence of neutron-pairs condensation on the nuclear symmetry energy slope

Highly linear correlation between the charge radii difference of mirror-pair nuclei and the slope parameter of symmetry energy has been built in the existing literatures. In this work, the impact of neutron-proton correlation deduced from the neutron and proton pairs condensation around the Fermi surface on determining the slope parameter of nuclear symmetry energy is investigated based on the Skyrme density functionals. The differential charge radii of Ni isotopes are employed to inspect the validity of this recently developed model. The calculated results suggest that the modified model can reproduce the shell quenching of charge radii at the neutron number $N=28$ along Ni isotopic chain. The shell closure effect of the charge radii can also be predicted at the neutron number $N=50$. The correlations between the charge radii differences of mirror partner nuclei $^{32}$Ar-$^{32}$Si and $^{54}$Ni-$^{54}$Fe and the slope parameters of symmetry energy are also analyzed. It is shown that the covered range of the symmetry energy slope is influenced by the neutron-pairs condensation around the Fermi surface. Moreover, a relatively stiff equation of state can be inferred from the mirror pairs $^{32}$Ar-$^{32}$Si and $^{54}$Ni-$^{54}$Fe when the influence coming from the neutron-pairs condensation is taken into account.