The interplay between different ordered phases, such as superconducting, charge or spin ordered phases, is of central interest in condensed-matter physics. The very recent discovery of superconductivity with a remarkable Tc = 26 K in Fe-based oxypnictide La(O1−xFx)FeAs (see Kamihara Y. et al., J. Am. Chem. Soc., 130 (2008) 3296) is a surprise to the scientific community and has generated tremendous interest. The pure LaOFeAs itself is not superconducting but shows an anomaly near 150 K in both resistivity and dc magnetic susceptibility. Here we provide combined experimental and theoretical evidences showing that a spin-density-wave (SDW) state develops at low temperature, in association with electron Nematic order. The electron-doping by F suppresses the SDW instability and induces the superconductivity. Therefore, the La(O1−xFx)FeAs offers an exciting new system showing competing orders in layered compounds.

Abstract:Here we report a new quaternary iron-arsenide superconductor Nd[O 1-x F x ]FeAs, with the onset resistivity transition at 51.9 K and Meissner transition at 51 K. This compound has the same crystal structure as LaOFeAs, and becomes the second superconductor after Pr[O 1-x F x ]FeAs that superconducts above 50 K.