Studying isotonic odd-even staggering of nuclear charge radii via spectroscopy of highly charged Yb and Lu

The nuclear charge radius is a fundamental observable that serves as an input to precision atomic structure calculations and a benchmark for state-of-the-art nuclear theories. In the highly deformed region between Eu ($Z=63$) and Hf ($Z=72$), traditional techniques (elastic electron scattering, muonic atom spectroscopy) for determining the charge radius are complicated by the interaction with the deformed nucleus, resulting in tension in reported values. We present a determination of the scaled nuclear charge radius difference between naturally abundant Yb ($Z=70$) and Lu ($Z=71$) using spectroscopy of highly charged ions, a technique that is relatively insensitive to nuclear deformation. This result supports the existence of a strong odd-even staggering (OES) in nuclear charge radius along isotonic chains of deformed nuclides. Additionally, it resolves an anomaly in the recommended nuclear charge radius of Lu, eliminating the apparent inversion in OES of charge radii along the Lu-Yb-Hf triplet. Results are also compared to nuclear density functional theory calculations performed using Skyrme and Fayans functionals.