A series of layered CeO1−xFxFeAs compounds with x=0 to 0.20 are synthesized by 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. The very high superconducting transition temperature strongly challenges the classic BCS theory based on the electron-phonon interaction. The very closeness of the superconducting phase to the spin-density-wave instability suggests that the magnetic fluctuations play a key role in the superconducting paring mechanism. The study also reveals that the Ce 4f electrons form local moments and ordered antiferromagnetically below 4 K, which could coexist with superconductivity.PACS numbers: 74.62.Bf, 74.25.Gz The recent discovery of superconductivity with transition temperature of 26 K in LaO 1−x F x FeAs system[1] has generated tremendous interest in the scientific community. Except for a relatively high transition temperature, the system displays many interesting properties. Among others, the presence of competing ordered ground states is one of the most interesting phenomena [2]. The pure LaOFeAs itself is not superconducting but shows an anomaly near 150 K in both resistivity and dc magnetic susceptibility.[1] This anomaly was shown to be caused by the spin-density-wave (SDW) instability.[2] Electrondoping by F suppresses the SDW instability and recovers the superconductivity. Here we show that similar competing orders exist in another rear-earth transition metal oxypnictide Ce(O 1−x F x )FeAs. Most surprisingly, the superconducting transition temperature in this system could reach as high as 41 K. Except for cuprate superconductors, T c in such iron-based compounds has already become the highest.The very high superconducting transition temperature has several important implications. First, the T c value has already reached the well-accepted limit value of classic BCS theory [3,4]. Considering the small carrier density and rather week electron-phonon coupling estimated from first-principle calculations [5,6], the observation result strongly challenges the BCS theory based on the electron-phonon interaction. Second, the rare-earth Cebased compounds usually show hybridization between localized f-electrons and itinerant electrons. This often leads to a strong enhancement of carrier effective mass at low temperature. Even for 4d transition metal oxypnictide with the same type of structure, a recent report indicates that the electronic specific heat coefficient of Ce-based CeORuP (γ=77 mJ/mol K 2 ) is 20 times higher than the value of La-based LaORuP (γ=3.9 mJ/mol K 2 ) [7]. The hybridization also tends to cause various ordered states at low temperature, like ferromagnetic (FM) or antiferromagnetic (AFM) ordering. Although superconducting state could occur in Ce-based materials, the superconducti...

A high quality superconducting Li 0.68 NbO 2 polycrystalline sample was prepared by deintercalation of Li ions from Li 0.93 NbO 2 . The field-dependent resistivity and specific heat were measured down to 0.5 K. The temperature dependence of the upper critical field is determined. A notable specific-heat jump is observed at the superconducting transition temperature T c ϳ 5.0 K at zero field. Below T c , the electronic specific heat shows a thermal activated behavior and agrees well with the theoretical result for the BCS s-wave superconductor. It indicates that the superconducting pairing in Li 0.68 NbO 2 has s-wave symmetry.