Superconductivity at 41 K and Its Competition with Spin-Density-Wave Instability in LayeredCeO1−xFxFeAs

A series of layered ${\mathrm{CeO}}_{1-x}{\mathrm{F}}_x\mathrm{FeAs}$ compounds with $x=0$ to 0.20 are synthesized by the solid state reaction method. Similar to the LaOFeAs, the pure CeOFeAs shows a strong resistivity anomaly near 145 K, which was ascribed to the spin-density-wave instability. $F$ doping suppresses this instability and leads to the superconducting ground state. Most surprisingly, the superconducting transition temperature could reach as high as 41 K. Such a high $T_c$ strongly challenges the classic BCS theory based on the electron-phonon interaction. The closeness of the superconducting phase to the spin-density-wave instability suggests that the magnetic fluctuation plays a key role in the superconducting pairing mechanism. The study also reveals that the Ce $4f$ electrons form local moments and are ordered antiferromagnetically below 4 K, which could coexist with superconductivity.