Upper critical field and thermally activated flux flow in single   crystalline Tl$_{0.58}$Rb$_{0.42}$Fe$_{1.72}$Se$_2$
  The upper critical field $\mu_0H_{c2}(T_c)$ of Tl$_{0.58}$Rb$_{0.42}$Fe$_{1.72}$Se$_2$ single crystals has been determined by means of measuring the electrical resistivity in both a pulsed magnetic field ($\sim$60T) and a DC magnetic field ($\sim$14T). It is found that $H_{c2}$ linearly increases with decreasing temperature for $\textbf{H}$$\parallel$$c$, reaching $\mu_0H_{c2}^{\textbf{H}\parallel c}(0\textrm{K})\simeq60$ T. On the other hand, a larger $\mu_0H_{c2}(0\textrm{K})$ with a strong convex curvature is observed for $\textbf{H}$$\perp$$c$ ($\mu_0H_{c2}^{\textbf{H}\perp c}$(18K)$\simeq$60T). This compound shows a moderate anisotropy of the upper critical field around $T_c$, but decreases with decreasing temperature. Analysis of the upper critical field based on the Werthamer-Helfand-Hohenberg (WHH) method indicates that $\mu_0H_{c2}(0\textrm{K})$ is orbitally limited for $\textbf{H}$$\parallel$$c$, but the effect of spin paramagnetism may play an important role on the pair breaking for $\textbf{H}$$\perp$$c$. All these experimental observations remarkably resemble those of the iron pnictide superconductors, suggesting a unified scenario for the iron-based superconductors. Moreover, the superconducting transition is significantly broadened upon applying a magnetic field, indicating strong thermal fluctuation effects in the superconducting state of Tl$_{0.58}$Rb$_{0.42}$Fe$_{1.72}$Se$_2$. The derived thermal activation energy for vortex motion is compatible with those of the 1111-type iron pnictides. 
L. Jiao, Y. Kohama, J. L. Zhang, H. D. Wang, B. Maiorov, F. F.   Balakirev, Y. Chen, L. N. Wang, T. Shang, M. H. Fang, and H...
cond-mat.supr-con