Charge radii of Sn isotopes in the relativistic mean field approximation

The kink observed in the nuclear charge radius of Sn isotopes around neutron number $N=82$ is investigated within the relativistic mean-field (RMF) framework using the NL3${}^{\ast }$ parameter set. It is shown that the small components of the Dirac spinors for the neutron single-particle states near the Fermi level play a crucial role in forming the kink through their contribution to the proton central potential. In particular, the significant differences between the radial parts of the small components of spin-orbit partner states make neutrons with $j=l-1\mathrm{/}2$ more efficient in increasing the nuclear charge radius than those with $j=l+1\mathrm{/}2$. However, the effect induced by the small components alone does not fully account for the magnitude of the kink observed in Sn isotopes.