The upper critical field and its anisotropy in (Li_1−xFe_x)OHFe_1−ySe

The temperature dependence of the upper critical field (H_c2) in a (Li_1−xFe_x)OHFe_1−ySe single crystal (T_c ≈ 40 K) has been determined by means of magnetotransport measurements down to 1.4 K both for inter-plane (H‖c, H^‖c _c2) and in-plane (H‖ab, H^‖ab _c2) field directions in static magnetic fields up to 14 T and pulsed magnetic fields up to 70 T. H^‖c _c2 exhibits a quasilinear increase with decreasing temperature below the superconducting transition and can be described well by an effective two-band model with unbalanced diffusivity, while H^‖ab _c2 shows a flattening below 35 K and follows the Werthamer–Helfand–Hohenberg (WHH) model incorporating orbital pair-breaking and spin-paramagnetic effects, yielding zero-temperature critical fields of H^‖c _c2(0) ≈ 67 T and H^‖ab _c2(0) ≈ 98 T. The anisotropy of the upper critical fields, γ(T)= H^‖ab _c2/ H^‖c _c2 monotonically decreases with decreasing temperature from about 7 near T_c to 1.5 at 0 K. This reduced anisotropy, observed in most Fe-based superconductors, is caused by the Pauli limitation of H^‖ab _c2.