Nonlinear isotope-shift effects in Be-like, B-like, and C-like argon

Nonlinear effects of the King plot in the isotope shifts of the 2$P$ fine-structure transitions in Be-, B-, and C-like argon ions are investigated through large-scale configuration-interaction calculations. Nonlinearities are found to be four orders of magnitude larger than previous estimates in comparable systems. This work could be important for the identification of possible nonlinearities originating from physics beyond the standard model.