Superconductivity suppression of Ba 0.5 K 0.5 Fe 2−2x M 2x As 2 single crystals by substitution of transition-metal (M = Mn, Ru, Co, Ni, Cu, and Zn) 5 Department of Chemistry, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588, Japan. 6 Department of Physics, Nagoya University, Furo-cho, Nagoya 464-8602, Japan.(Dated: June 6, 2012)We investigated the doping effects of magnetic and nonmagnetic impurities on the singlecrystalline p-type Ba0.5K0.5Fe2−2xM 2xAs2 (M = Mn, Ru, Co, Ni, Cu and Zn) superconductors. The superconductivity indicates robustly against impurity of Ru, while weakly against the impurities of Mn, Co, Ni, Cu, and Zn. However, the present T c suppression rate of both magnetic and and nonmagnetic impurities remains much lower than what was expected for the s±-wave model. The temperature dependence of resistivity data is observed an obvious low-T upturn for the crystals doped with high-level impurity, which is due to the occurrence of localization. Thus, the relatively weak T c suppression effect from Mn, Co, Ni, Cu, and Zn are considered as a result of localization rather than pair-breaking effect in s±-wave model.

We present magnetization, specific heat, resistivity, and Hall effect measurements on the cubic B20 phase of MnGe and CoGe and compare to measurements of isostructural FeGe and electronic-structure calculations. In MnGe, we observe a transition to a magnetic state at T c = 275 K as identified by a sharp peak in the ac magnetic susceptibility, as well as second phase transition at lower temperature that becomes apparent only at finite magnetic field. We discover two phase transitions in the specific heat at temperatures much below the Curie temperature, one of which we associate with changes to the magnetic structure. A magnetic field reduces the temperature of this transition which corresponds closely to the sharp peak observed in the ac susceptibility at fields above 5 kOe. The second of these transitions is not affected by the application of field and has no signature in the magnetic properties or our crystal-structure parameters. Transport measurements indicate that MnGe is metallic with a negative magnetoresistance similar to that seen in isostructural FeGe and MnSi. Hall effect measurements reveal a carrier concentration of about 0.5 carriers per formula unit, also similar to that found in FeGe and MnSi. CoGe is shown to be a low carrier density metal with a very small, nearly temperature-independent diamagnetic susceptibility.

Additional charge carriers were introduced to the iron oxyarsenide Sr 4 Sc 2 Fe 2 As 2 O 6 under a high-pressure condition, followed by measurements of electrical resistivity, Hall coefficient, and magnetic susceptibility. The host compound Sr 4 Sc 2 Fe 2 As 2 O 6 shows metallic conductivity down to ~200 K and turns to show a semiconducting-like conductivity accompanied by a positive magneto-resistance (22% at 70 kOe). Although the carrier density is comparable at 300 K (5.9×10 21 cm -3 ) with that of the other Fe-based superconductors, no superconductivity appears down to 2 K. This is primarily because the net carrier density decreases over 3 orders of magnitude on cooling and additionally a possible magnetic order at ~120 K prevents carriers from pairing. The properties were altered largely by introducing the additional carriers.

A dense phase of a postlayered perovskite Sr 3 Co 2 O 6 was created by a high-pressure and high-temperature synthetic route and quenched to ambient conditions. The structure of Sr 3 Co 2 O 6 was determined by synchrotron x-ray powder diffraction. Sr 3 Co 2 O 6 crystallizes in a K 4 CdCl 6 -type structure (an infinite chain-type structure), which is 6% denser than the known isocompositional layered perovskite Sr 3 Co 2 O 7−δ (δ ∼ 1). The K 4 CdCl 6 -type Sr 3 Co 2 O 6 shows complex magnetic behavior, reflecting its spin-chain nature, as do the analog oxides Ca 3 Co 2 O 6 and Ca 3 CoRhO 6 , whose microscopic magnetic origin is still highly debated.