Upper critical field and thermally activated flux flow in single-crystalline Tl _0.58Rb _0.42Fe _1.72Se ₂

The upper critical field μ ₀H _c2(T _c) of Tl _0.58Rb _0.42Fe _1.72Se ₂ single crystals has been determined by means of measuring the electrical resistivity in both a pulsed magnetic field (∼58T) and a dc magnetic field (∼14T). It is found that μ ₀H _c2 linearly increases with decreasing temperature for H-c, reaching μ ₀Hc2Hc(0K) 60T. On the other hand, a larger μ ₀H _c2(0K) with a strong convex curvature is observed for Hc[μ ₀Hc2Hc(18K) 60T]. This compound shows a moderate anisotropy of the upper critical field around T _c, which decreases with decreasing temperature. Analysis of the upper critical field based on the Werthamer-Helfand-Hohenberg (WHH) method indicates that μ ₀H _c2(0K) is orbitally limited for Hc, but the effect of spin paramagnetism may play an important role in the pair breaking for Hc. All these experimental observations remarkably resemble those of the iron pnictide superconductors, suggesting a universal scenario for the iron-based superconductors. Moreover, the superconducting transition is broadened significantly upon applying a magnetic field, indicating strong thermal fluctuation effects in the superconducting state of Tl _0.58Rb _0.42Fe _1.72Se ₂. The derived thermal activation energy for vortex motion is compatible with those of the 1111-type iron pnictides.