Synthesis and physical properties of LaO 1-x F x FeAs

We have prepared the newly discovered Fe-based superconducting material LaO 1-x F x FeAs (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 T s ≈ 160 K and antiferromagnetic spin order at T N ≈ 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 to x ≈ 0.04. However, the transitions are abruptly suppressed at x ≥ 0.05 in favour of a superconducting phase with a critical temperature T c ≳ 20 K. Upon further increasing the F-doping T c increases up to a maximum of T c = 26.8 K at x = 0.1 which is followed by a decrease down to T c ≈ 10 K at x ≥ 0.15.