Here we report the discovery of superconductivity in the ternary LaRu 2 As 2 compound. The polycrystalline LaRu 2 As 2 samples were synthesized by the conventional solid state reaction method. Powder X-ray diffraction analysis indicates that LaRu 2 As 2 crystallizes in the ThCr 2 Si 2 -type crystal structure with the space group I4/mmm (No. 139), and the refined lattice parameters are a = 4.182(6) Å and c = 10.590(3) Å. 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  0 H 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 2 As 2 is a type-II superconductor.

The (Ca,R)FeAs 2 (R=La,Pr and etc.) superconductors with a signature of superconductivity transition above 40 K possess a new kind of block layers that consist of zig-zag As chains. In this paper, we report the electronic structure of the new (Ca,La)FeAs 2 superconductor investigated by both band structure calculations and high resolution angle-resolved photoemission spectroscopy measurements. Band structure calculations indicate that there are four hole-like bands around the zone center Γ(0,0) and two electron-like bands near the zone corner M(π,π) in CaFeAs 2 . In our angle-resolved photoemission measurements on (Ca 0.9 La 0.1 )FeAs 2 , we have observed three holelike bands around the Γ point and one electron-like Fermi surface near the M(π,π) point. These results provide important information to compare and contrast with the electronic structure of other iron-based compounds in understanding the superconductivity mechanism in the iron-based superconductors.1 arXiv:1311.6042v1 [cond-mat.supr-con]

Single crystals of undoped CaFe 2 As 2 were grown by a FeAs self-flux method, and the crystals were quenched in ice-water rapidly after high temperature growth. The quenched crystal undergoes a collapsed tetragonal structural phase transition around 80 K revealed by the temperature dependent X-ray diffraction measurements.Superconductivity below 25 K was observed in the collapsed phase by resistivity and magnetization measurements. The isothermal magnetization curve measured at 2 K indicates that this is a typical type-II superconductor. For comparison, we systematically characterized the properties of the furnace cooled, quenched, and post-annealed single crystals, and found strong internal crystallographic strain existing in the quenched samples, which is the key for the occurrence of superconductivity in the undoped CaFe 2 As 2 single crystals.