Upper critical field and thermally activated flux flow in single crystalline Tl$_{0.58}$Rb$_{0.42}$Fe$_{1.72}$Se$_2$

TL;DR

This study investigates the upper critical field and flux flow behavior in Tl$_{0.58}$Rb$_{0.42}$Fe$_{1.72}$Se$_2$ single crystals, revealing anisotropic superconducting properties, fluctuation effects, and similarities with iron pnictide superconductors.

Contribution

It provides detailed measurements of the upper critical field and flux flow in Tl$_{0.58}$Rb$_{0.42}$Fe$_{1.72}$Se$_2$, highlighting anisotropy and fluctuation effects, and compares these with other iron-based superconductors.

Findings

Upper critical field reaches ~60 T for both field orientations.

Anisotropy decreases with decreasing temperature.

Strong thermal fluctuations influence the superconducting transition.

Abstract

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.