Upper critical magnetic field of KxFe2−ySe2and Eu0.5K0.5Fe2As2single crystals

Abstract

The $H\ensuremath{-}T$ phase diagrams of single crystalline electron-doped ${\text{K}}_{0.83}{\text{Fe}}_{1.83}{\text{Se}}_{2}$ (KFS1), ${\text{K}}_{0.8}{\text{Fe}}_{2}{\text{Se}}_{2}$ (KFS2) and hole-doped ${\text{Eu}}_{0.5}{\text{K}}_{0.5}{\text{Fe}}_{2}{\text{As}}_{2}$ (EKFA) have been deduced from tunnel diode oscillator-based contactless measurements in pulsed magnetic fields up to $57\phantom{\rule{4.pt}{0ex}}\text{T}$ for the interplane ($H\ensuremath{\parallel}c$) and in-plane ($H\ensuremath{\parallel}ab$) directions. The temperature dependence of the upper critical magnetic field ${H}_{c2}(T)$ relevant to EFKA is accounted for by Pauli model including an anisotropic Pauli paramagnetic contribution (${\ensuremath{\mu}}_{B}{H}_{p}=114\phantom{\rule{4.pt}{0ex}}\text{T}$ for $H\ensuremath{\parallel}ab$ and $86\phantom{\rule{4.pt}{0ex}}\text{T}$ for $H\ensuremath{\parallel}c$). This is also the case of KFS1 and KFS2 for $H\ensuremath{\parallel}ab$ whereas a significant upward curvature, accounted for by a two-gap model, is observed for $H\ensuremath{\parallel}c$. Despite the presence of antiferromagnetic lattice order within the superconducting state of the studied compounds, no influence of magnetic ordering on the temperature dependence of ${H}_{c2}(T)$ is observed.