Upper critical field and thermally activated flux flow in single-crystalline Tl0.58Rb0.42Fe1.72Se2

The upper critical field mu H-0(c2)(T-c) of Tl0.58Rb0.42Fe1.72Se2 single crystals has been determined by means of measuring the electrical resistivity in both a pulsed magnetic field (similar to 58 T) and a dc magnetic field (similar to 14 T). It is found that mu H-0(c2) linearly increases with decreasing temperature for H parallel to c, reaching mu H-0(c2)H parallel to c (0 K) similar or equal to 60 T. On the other hand, a larger mu H-0(c2)(0 K) with a strong convex curvature is observed for H perpendicular to c [mu H-0(c2)H perpendicular to c (18 K) similar or equal to 60 T]. 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 mu H-0(c2)(0 K) is orbitally limited for H parallel to c, but the effect of spin paramagnetismmay play an important role in the pair breaking for H perpendicular to c. 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 Tl0.58Rb0.42Fe1.72Se2. The derived thermal activation energy for vortex motion is compatible with those of the 1111-type iron pnictides.