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.

By substituting Fe with the 5d-transition metal Pt in BaFe2As2, we have successfully synthesized the superconductors BaFe2−xPtxAs2. The systematic evolution of the lattice constants indicates that the Fe ions were successfully replaced by Pt ions. By increasing the doping content of Pt, the antiferromagnetic order and structural transition of the parent phase is suppressed and superconduc-tivity emerges at a doping level of about x = 0.02. At a doping level of x = 0.1, we get a maximum transition temperature Tc of about 25 K. While even for this optimally doped sample, the residual resistivity ratio (RRR) is only about 1.35, indicating a strong impurity scattering effect. We thus argue that the doping to the Fe-sites naturally leads to a high level impurity scattering, although the superconductivity can still survive at about 25 K. The synchrotron powder x-ray diffraction shows that the resistivity anomaly is in good agreement with the structural transition. The super-conducting transitions at different magnetic fields were also measured at the doping level of about x = 0.1, yielding a slope of-dHc2/dT = 5.4 T/K near Tc. Finally a phase diagram was established for the Pt doped 122 system. Our results suggest that superconductivity can also be easily induced in the FeAs family by substituting the Fe with Pt, with almost the similar maximum transition temperatures as doping Ni, Co, Rh and Ir.

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.

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We have successfully synthesized one member of LnFeAsO (Ln stands for lanthanides) with Ln=Tb. By partial substitution of Tb 3+ by Th 4+ , superconductivity with onset Tc up to 52 K was observed. In the undoped parent compound, the magnetic moments of Tb 3+ ions order antiferromagnetically (AFM) at TN of 2.5 K, and there exists an anomaly in the resistivity at T * of about 124 K which corresponds to the structural phase transition and/or AFM ordering of magnetic moments of Fe 2+ ions. This anomaly is severely suppressed by Th doping, similar to the cases of F doped LnFeAsO series. Thermopower measurements show that the charge carrier is electron-like for both undoped and Th doped compounds, and enhanced thermopower in Th doped samples was observed.

We present a systematic study on the physical properties of EuFe2−xNixAs2 (0≤x ≤0.2) by electrical resistivity, magnetic susceptibility and thermopower measurements. The undoped compound EuFe2As2 undergoes a spin-density-wave (SDW) transition associated with Fe moments at 195 K, followed by antiferromagnetic (AFM) ordering of Eu 2+ moments at 20 K. Ni doping at the Fe site simultaneously suppresses the SDW transition and AFM ordering of Eu 2+ moments. For x ≥0.06, the magnetic ordering of Eu 2+ moments evolves from antiferromagnetic to ferromagnetic (FM). The SDW transition is completely suppressed for x ≥0.16, however, no superconducting transition was observed down to 2 K. The possible origins of the AFM-to-FM transition and the absence of superconductivity in EuFe2−xNixAs2 system are discussed.