Upper critical magnetic field of K_xFe_(2-y)Se_2 and Eu_(0.5)K_(0.5)Fe_2As_2 single crystals

摘要

The H-T phase diagrams of single crystalline electron-doped K_(0.83)Fe_(1.83)Se_2 (KFS1), K_(0.8)Fe_2Se_2 (KFS2) and hole-doped Eu_(0.5)K_(0.5)Fe_2As_2 (EKFA) have been deduced from tunnel diode oscillator-based contactless measurements in pulsed magnetic fields up to 57 T for the interplane (H ‖ c) and in-plane (H ‖ 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 (μ_BH_p = 114 T for H ‖ ab and 86 T for H ‖ c). This is also the case of KFS1 and KFS2 for H ‖ ab whereas a significant upward curvature, accounted for by a two-gap model, is observed for H ‖ 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_ ca)(T) is observed.