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 ${\mu }_0{H}_{c2}(T_c)$ 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 $\mathbf{\text{H}}$$\parallel$$c$, reaching ${\mu }_0{H}_{c2}^{\mathbf{\text{H}}\parallel c}(0\text{K})\simeq 60$ T. On the other hand, a larger ${\mu }_0{H}_{c2}(0\text{K})$ with a strong convex curvature is observed for $\mathbf{\text{H}}$$\perp$$c$ ($\mu_0H_{c2}^{$\mathbf{\text{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 ${\mu }_0{H}_{c2}(0\text{K})$ is orbitally limited for $\mathbf{\text{H}}$$\parallel$$c$, but the effect of spin paramagnetism may play an important role on the pair breaking for $\mathbf{\text{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.