  A systematic global investigation of differential charge radii has been
performed within the CDFT framework for the first time. Theoretical results
obtained with conventional covariant energy density functionals and separable
pairing interaction are compared with experimental differential charge radii in
the regions of the nuclear chart in which available experimental data crosses
neutron shell closures at N = 28, 50, 82 and 126. The analysis of absolute
differential radii of different isotopic chains and their relative properties
indicate clearly that such properties are reasonably well described in model
calculations in the cases when the mean-field approximation is justified.
However, while the observed clusterization of differential charge radii of
different isotopic chains is well described above the N=50 and N=126 shell
closures, it is more difficult to reproduce it above the N=28 and N=82 shell
closures because of possible deficiencies in underlying single-particle
structure. The impact of the latter has been evaluated for spherical shapes and
it was shown that the relative energies of the single-particle states and the
patterns of their occupation with increasing neutron number have an appreciable
impact on the evolution of the differential charge radii. It is shown that the
kinks in the charge radii at neutron shell closures are due to the underlying
single-particle structure and due to weakening or collapse of pairing at these
closures. It is usually assumed that pairing is a dominant contributor to
odd-even staggering (OES) in charge radii. Our analysis paints a more
complicated picture. It suggests a new mechanism in which the fragmentation of
the single-particle content of the ground state in odd-mass nuclei due to
particle-vibration coupling provides a significant contribution to OES in
charge radii.
