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...

Ir1−xPtxTe2 is an interesting system showing competing phenomenon between structural instability and superconductivity. Due to the large atomic numbers of Ir and Te, the spin-orbital coupling is expected to be strong in the system which may lead to nonconventional superconductivity. We grew single crystal samples of this system and investigated their electronic properties. In particular, we performed optical spectroscopic measurements, in combination with density function calculations, on the undoped compound IrTe2 in an effort to elucidate the origin of the structural phase transition at 280 K. The measurement revealed a dramatic reconstruction of band structure and a significant reduction of conducting carriers below the phase transition. We elaborate that the transition is not driven by the density wave type instability but caused by the crystal field effect which further splits/separates the energy levels of Te (px, py) and Te pz bands.

A large and high-quality single crystal (Li 0.84 Fe 0.16 )OHFe 0.98 Se, the optimal superconductor of newly reported (Li 1-x Fe x )OHFe 1-y Se system, has been successfully synthesized via a hydrothermal ion-exchange technique. The superconducting transition temperature (T c ) of 42 K is determined by magnetic susceptibility and electric resistivity measurements, and the zero-temperature upper critical magnetic fields are evaluated as 79 and 313 Tesla for the field along the c-axis and the ab-plane, respectively. The ratio of out-of-plane to in-plane electric resistivity,ρ c /ρ ab , is found to increases with decreasing temperature and to reach a high value of 2500 at 50 K, with an evident kink occurring at a characteristic temperature T*=120 K. The negative in-plane Hall coefficient indicates that electron carriers dominate in the charge transport, and the hole contribution is significantly reduced as the temperature is lowered to approach T*. From T* down to T c , we observe the linear temperature dependences of the in-plane electric resistivity and the magnetic susceptibility for the FeSe layers. Our findings thus reveal that the normal state of (Li 0.84 Fe 0.16 )OHFe 0.98 Se becomes highly two-dimensional and anomalous prior to the superconducting transition, providing a new insight into the mechanism of high-T c superconductivity.