Vortex phase transition and anisotropy behavior of optimized (Li1−x Fe x OH)FeSe single crystals.

(Li_1−xFe_xOH)FeSe single crystals have been successfully synthesized by a hydrothermal ion-exchange technique. We have carried out systematic research on the influence of the concentration of selenourea (c_m) on the superconducting properties of (Li_1−xFe_xOH)FeSe single crystals. The optimized specimens possess an onset superconducting transition temperature T_c of up to 42 K obtained at c_m = 0.8 mol l⁻¹, and the corresponding residual resistivity ratio (RRR = ρ (300 K)/ρ (T_c)) is estimated to be 11. The vortex liquid-to-glass phase transition has been discussed based on the collective pinning model. The large distance between the H_g and H_c2 lines, as well as the much narrower region of the pinned vortex-liquid phase, indicate a weak vortex-pinning ability in (Li_1−xFe_xOH)FeSe single crystals. The big out-plane anisotropy behavior versus temperature has been explored according to the anisotropic Ginzburg–Landau theory, and the anisotropy factor Γ displays linear behavior towards temperature similar to that of SmFeAsO_1−xF_x superconductors. [ABSTRACT FROM AUTHOR]