The electronic phase diagrams of the Eu(Fe0.81Co0.19)2As2 superconductor

Abstract

The magnetic and superconducting properties of an Eu(Fe0.81Co0.19)2As2 single crystal are investigated by means of ac magnetic susceptibility, dc magnetization, specific heat, transverse resistivity and Hall effect measurements in magnetic fields up to 9 T, applied parallel and perpendicular to the c-axis. The compound exhibits the coexistence of magnetism and superconductivity (SC), characterized by structural distortion (SD) and/or spin-density-wave (SDW) ordering at TSD/SDW = 78 ± 4 K, canted-antiferromagnetic (C-AF) ordering at the Néel temperature TN = 16.5 ± 0.5 K and SC at the critical temperature Tc = 5.3 ± 0.2 K at zero field. Upon applying fields both the C-AF and SC states evolve in an unconventional manner. Magnetic field distinctly affects the spin canting, resulting in separation of the C-AF into two new phases: the C-AF and ferromagnetic (F) ones. The unusual behavior of the SC state produces field-induced SC in the H⊥c configuration as an outcome of the weakening orbital pair-breaking effect. From the experimental data we derive the field-temperature phase diagrams for Eu(Fe0.81Co0.19)2As2. A comparison of experimental results is made with theory developed for type II superconductors and then some important thermodynamic parameters characteristic of the superconducting state of Eu(Fe0.81Co0.19)2As2 are deduced such as the specific heat jump at Tc, ΔCp(Tc)/γnTc, the electron–phonon coupling constant λe–ph, the upper critical field Hc2, coherence length ξ, the Fermi wave-vector kF, effective mass m*, Hall mobility μH, magnetic penetration depth λ and the Ginzburg–Landau parameter κ.