Synthesis and physical properties of LaO1−xFxFeAs

We have prepared the newly discovered Fe-based superconducting material LaO1−xFxFeAs (0 ≤ x ≤ 0.2) in polycrystalline form and have investigated the samples by means of structural, thermodynamic and transport measurements. Our investigations reveal a non superconducting phase at 0 ≤ x . 0.04 which for x = 0 is characterized by a structural transition towards an orthorhombic distortion at Ts ≈ 160 K and antiferromagnetic spin order at TN ≈ 138 K. Both transitions lead to strong anomalies in various transport properties as well as in magnetization and in specific heat. Remarkably, the transition temperatures are only weakly doping dependent up x . 0.04. However, the transitions are abruptly suppressed at x ≥ 0.05 in favour of a superconducting phase with a critical temperature Tc & 20 K. Upon further increasing the F-doping Tc increases up to a maximum of Tc = 26.8 K at x = 0.1 which is followed by a decrease down to Tc ≈ 10 K at x ≥ 0.15. PACS. 74.70.-b Superconducting materials – 74.25.Bt Thermodynamic properties – 74.25.Fy Transport properties (electric and thermal conductivity, thermoelectric effects, etc.)