Doping dependence of resistivity, upper critical field and its anisotropy in overdoped Ba_1–xK_xFe₂As₂ (x = 0.6–1) single crystals

Temperature-dependent resistivity, upper critical field H_c2 and its anisotropy in overdoped superconducting Ba_1−xK_xFe₂As₂ (x = 0.6–1) single crystals have been measured in steady magnetic fields up to 44 T and low temperatures down to 0.4 K. Analysis using both the quadratic term and power-law fitting demonstrates that the in-plane resistivity ρ_ab(T) progressively approaches the Fermi-liquid T² behavior with increasing K doping and reaches a saturation plateau at x ≈ 0.8. The temperature dependence of both $H_{\mathrm{c}2}^{ab}$ and $H_{\mathrm{c}2}^c$ follows the Werthamer–Helfand–Hohenberg model, incorporating orbital and spin paramagnetic effects. For x ≤ 0.8, the orbital effect dominates for H ∥ ab, while the Pauli paramagnetic effect prevails for H ∥ c. For x > 0.8, the Pauli paramagnetic effect becomes dominant in both crystallographic directions. The anisotropy of H_c2(0) exhibits a discontinuity in its dependence on K doping concentration with a significant enhancement at x = 0.8 and a maximum at x = 0.9. These experimental results indicate that the electron correlation effect is enhanced in the heavily overdoped Ba_1−xK_xFe₂As₂ system where the underlying symmetries are broken due to the Fermi surface reconstruction before x = 0.9.