Nuclear charge radii of germanium isotopes around N = 40

Collinear laser spectroscopy measurements were performed on ⁶⁸⁻⁷⁴Ge isotopes ($Z = 32$) at ISOLDE-CERN, by probing the 4s² 4p² \, ³\!P₁ → 4s² 4p 5s \, ³\!P₁ᵒ atomic transition (269~nm) of germanium. Nuclear charge radii are determined via the measured isotope shifts, revealing a larger local variation than the neighboring isotopic chains. Nuclear density functional theory with the Fayans functionals Fy(Δ r,HFB) and Fy(IVP), and the SV-min Skyrme describes the experimental data for the differential charge radii δ r² and charge radii Rc within the theoretical uncertainties. The observed large variation in the charge radii of germanium isotopes is better accounted for by theoretical models incorporating ground state quadrupole correlations. This suggests that the polarization effects due to pairing and deformation contribute to the observed large odd-even staggering in the charge radii of the Ge isotopic chain.