We report a new strategy to induce superconductivity in iron-based oxyarsenide. Instead of F − substitution for O 2− , we employed Th 4+ doping in GdFeAsO with the consideration of "lattice match" between Gd2O2 layers and Fe2As2 ones. As a result, superconductivity with T onset c as high as 56 K was realized in a Gd0.8Th0.2FeAsO polycrystalline sample. This Tc value is among the highest ever discovered in the iron-based oxypnictides.

We report bulk superconductivity induced by an isovalent doping of phosphorus in BaFe(2)(As(1-x)P(x))(2). The P-for-As substitution results in shrinkage of the lattice, especially for the FeAs block layers. The resistivity anomaly associated with the spin-density-wave (SDW) transition in the undoped compound is gradually suppressed by the P doping. Superconductivity with a maximum T(c) of 30 K emerges at x = 0.32, coinciding with a magnetic quantum critical point (QCP) which is shown by the disappearance of SDW order and the linear temperature-dependent resistivity in the normal state. The T(c) values were found to decrease with further P doping and no superconductivity was observed down to 2 K for x≥0.77. The appearance of superconductivity in the vicinity of QCP hints at the superconductivity mechanism in iron-based arsenides.

We have studied EuFe2(As0.7P0.3)2 by the measurements of x-ray diffraction, electrical resistivity, thermopower, magnetic susceptibility, magnetoresistance and specific heat. Partial substitution of As with P results in the shrinkage of lattice, which generates chemical pressure to the system. It is found that EuFe2(As0.7P0.3)2 undergoes a superconducting transition at 26 K, followed by ferromagnetic ordering of Eu 2+ moments at 20 K. This finding is the first observation of superconductivity stabilized by internal chemical pressure, and supplies a rare example showing coexistence of superconductivity and ferromagnetism in the ferro-arsenide family.

Ternary iron arsenide EuFe2As2 with ThCr2Si2-type structure has been studied by magnetic susceptibility, resistivity, thermopower, Hall and specific heat measurements. The compound undergoes two magnetic phase transitions at about 200 K and 20 K, respectively. The former was found to be accompanied with a slight drop in magnetic susceptibility (after subtracting the Curie-Weiss paramagnetic contribution), a rapid decrease in resistivity, a large jump in thermopower and a sharp peak in specific heat with decreasing temperature, all of which point to a spin-density-wave-like antiferromagnetic transition. The latter was proposed to be associated with an A-type antiferromagnetic ordering of Eu 2+ moments. Comparing with the physical properties of the iso-structural compounds BaFe2As2 and SrFe2As2, we expect that superconductivity could be induced in EuFe2As2 through appropriate doping. [18,19,20] EuFe 2 As 2 is another member of the ternary iron arsenide family, [21] however, only few work was performed on this material. Mössbauer and magnetic susceptibility studies [22] indicated that EuFe 2 As 2 experienced two magnetic transitions. The first one around 200 K was due to the AFM transition in the iron sublattice. The second one at 19 K arose from the AFM ordering of Eu 2+ magnetic moments. No other physical properties of EuFe 2 As 2 have been reported. In order to assess the potential of inducing superconductivity in this compound, we have carried out a systematic study of the physical properties of EuFe 2 As 2 . We found that the transition at about 200 K was accompanied by a rapid decrease in resistivity, a large jump in thermopower and a sharp peak in specific heat. In addition, a slight drop in magnetic susceptibility was observed after subtracting the Curie-Weiss paramagnetic contribution of Eu 2+ magnetic moments. These properties are quite similar with those of BaFe 2 As 2 and SrFe 2 As 2 , suggesting that EuFe 2 As 2 is another possible parent compound in which superconductivity may be found by proper doping. Polycrystalline samples of EuFe 2 As 2 were synthesized from stoichiometric amounts of the elements as reported previously [21]. Fresh Eu grains, Fe powders and As grains were mixed in a ratio of 1:2:2, sealed in an evacuated quartz tube and sintered at 773 K for 12 hours then 1073 K for another 12 hours. After cooling, the reaction product was thoroughly ground in an agate mortar and pressed into pellets under a pressure of 2000 kg/cm 2 in an argon-filled glove-box. The pellets were annealed in an evacuated quartz tube at 1123 K for 12 hours and furnace-cooled to room temperature. The EuFe 2 As 2 samples were obtained as black powders, which is stable in air.

Magnetic flux structure on the surface of EuFe2(As1−xPx)2 single crystals with nearly optimal phosphorus doping levels x = 0.20, and x = 0.21 is studied by low-temperature magnetic force microscopy and decoration with ferromagnetic nanoparticles. The studies are performed in a broad temperature range. It is shown that the single crystal with x = 0.21 in the temperature range between the critical temperatures TSC = 22 K and TC = 17.7 K of the superconducting and ferro-magnetic phase transitions, respectively has the vortex structure of a frozen magnetic flux, typical for type-II superconductors. The magnetic domain structure is observed in the superconducting state below TC. The nature of this structure is discussed.