The upper critical field and its anisotropy in (Li1-x Fe x )OHFe1-y Se.

The temperature dependence of the upper critical field (H c2) in a (Li1-x Fe x )OHFe1-y Se single crystal ([Formula: see text] K) has been determined by means of magnetotransport measurements down to 1.4 K both for inter-plane ([Formula: see text], [Formula: see text]) and in-plane ([Formula: see text], [Formula: see text]) field directions in static magnetic fields up to 14 T and pulsed magnetic fields up to 70 T. [Formula: see text] 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 [Formula: see text] 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 [Formula: see text] T and [Formula: see text] T. The anisotropy of the upper critical fields, [Formula: see text] 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 [Formula: see text].