Nuclear magnetic shielding in heliumlike ions

Ab initio QED calculations of the nuclear magnetic shielding constant in heliumlike ions are presented. We combine the nonrelativistic QED approach based on an expansion in powers of the fine -structure constant alpha and the so-called all -order QED approach, which includes all orders in the parameter Z alpha but uses a perturbation expansion in the parameter 1/Z (where Z is the nuclear charge number). The combination of the two complementary methods makes our treatment applicable to both low -Z and high -Z ions. Our calculations confirm the presence of a rare antiscreening effect for the relativistic shielding correction and demonstrate the importance of the inclusion of the negative -energy part of the Dirac spectrum.