Synthesis and physical properties of LaO1-xFxFeAs

We have prepared the newly discovered Fe-based superconducting material LaO_1-xF_xFeAs (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.