We report the discovery and characterization of a novel 112-type iron pnictide Eu Fe As_2, with La-doping induced superconductivity in a series of Eu_(1-x)La_xFeAs_2. The polycrystalline samples were synthesized through solid state reaction method only within a very narrow temperature window around 1073 K.Small single crystals were also grown from a flux method with the size about 100 lm. The crystal structure was identified by single crystal X-ray diffraction analysis as a monoclinic structure with space group of P21/m. From resistivity and magnetic susceptibility measurements, we found that the parent compound Eu Fe As_2 shows distinct anomalies probably due to the Fe^(2+) related antiferromagnetic/structural phase transition near 110 K and the Eu^(2+) related antiferromagnetic phase transition near 40 K.La-doping suppressed both phase transitions to lower temperatures and induced superconducting transitions with a Tc^11 K for Eu_(0.85)La_(0.15) Fe As_2.
The iron-based superconductivity (IBSC) is a great challenge in correlated system. Angle-resolved photoemission spectroscopy (ARPES) provides electronic structure of the IBSCs, the pairing strength, and the order parameter symmetry. Here, we briefly review the recent progress in IBSCs and focus on the results from ARPES. The ARPES study shows the electronic structure of "122", "111", "11", and "122 " families of IBSCs. It has been agreed that the IBSCs are unconventional superconductors in strong coupling region. The order parameter symmetry basically follows s ± form with considerable out-of-plane contribution.
Here we report the discovery of superconductivity in the ternary LaRu_2As_2 compound. The polycrystalline LaRu_2As_2 samples were synthesized by the conventional solid state reaction method. Powder X-ray diffraction analysis indicates that LaRu_2As_2 crystallizes in the Th Cr_2Si_2-type crystal structure with the space group I4/mmm(No. 139), and the refined lattice parameters are a = 4.182(6) and c = 10.590(3) . The temperature dependent resistivity measurement shows a clear superconducting transition with the onset T_c(critical temperature) at 7.8 K, and zero resistivity happens at 6.8 K. The upper critical field at zero temperature μ_0H_(c2)(0) was estimated to be 1.6 T from the resistivity measurement. DC magnetic susceptibility measurement shows a bulk superconducting Meissner transition at 7.0 K, and the isothermal magnetization measurement indicates that LaRu_2As_2 is a type-II superconductor.
We performed a combined angle-resolved photoemission spectroscopy and scanning tunneling microscopy study of the electronic structure of electron-doped Ca_(0.83)La_(0.17)Fe_(2)As_(2).A surface reconstruction associated with the dimerization of As atoms is observed directly in the real space,as well as the consequent band folding in the momentum space.Besides this band folding effect,the Fermi surface topology of this material is similar to that reported previously for BaFe_(1.85)Co_(0.15)As_(2),with Γ-centered hole pockets quasi-nested to M-centered electron pockets by the antiferromagnetic wave vector.Although no superconducting gap is observed by ARPES possibly due to low superconducting volume fraction,a gap-like density of states depression of 7.7±2.9 meV is determined by scanning tunneling microscopy.
