Upper critical magnetic field of KxFe2-ySe 2 and Eu0.5K0.5Fe2As2 single crystals

Abstract

The H-T phase diagrams of single crystalline electron-doped K 0.83Fe1.83Se2 (KFS1), K0.8Fe 2Se2 (KFS2) and hole-doped Eu0.5K 0.5Fe2As2 (EKFA) have been deduced from tunnel diode oscillator-based contactless measurements in pulsed magnetic fields up to 57T for the interplane (Hâ̂¥c) and in-plane (Hâ̂¥ ab) directions. The temperature dependence of the upper critical magnetic field Hc2(T) relevant to EFKA is accounted for by Pauli model including an anisotropic Pauli paramagnetic contribution (μBHp=114T for Hâ̂¥ab and 86T for Hâ̂¥c). This is also the case of KFS1 and KFS2 for Hâ̂¥ab whereas a significant upward curvature, accounted for by a two-gap model, is observed for Hâ̂¥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 Hc2(T) is observed. © 2013 American Physical Society.