The upper critical field and its anisotropy in ( Li 1 − x Fe x ) OHFe 1 −

The temperature dependence of the upper critical field (Hc2) in a (Li1−xFex)OHFe1−ySe single crystal (T 40 c≈ 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 0 67 c c2 ( ) // ≈ T and H 0 98 ab c2 ( ) // ≈ T. The anisotropy of the upper critical fields, T H H ab c c2 c2 ( ) / // // γ = monotonically decreases with decreasing temperature from about 7 near Tc 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 // .