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.

Here we report a new class of superconductors prepared by high pressure synthesis in the quaternary family ReFeAsO 1-d (Re = Sm, Nd, Pr, Ce, La) without fluorine doping. The onset superconducting critical temperature (T c ) in these compounds increases with the reduction of Re atom size, and the highest T c obtained so far is 55 K in SmFeAsO 1-d . For the NdFeAsO 1-d compound with different oxygen concentration a dome-shaped phase diagram was found.

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Ren Zhi-An (任治安)*, Lu Wei (陆伟), Yang Jie (杨杰), Yi Wei (衣玮), Shen Xiao-Li (慎晓丽), Li Zheng-Cai (李 正才), Che Guang-Can (车广灿), Dong Xiao-Li (董晓莉), Sun Li-Ling (孙立玲), Zhou Fang (周放), Zhao Zhong-Xian (赵忠贤)*

Early studies have found quasi-reversible magnetization curves in polycrystalline bulk rare-earth iron oxypnictides that suggest either wide-spread obstacles to intergranular current or very weak vortex pinning. In the present study of polycrystalline samarium and neodymium rare-earth iron oxypnictide samples made by high pressure synthesis, the hysteretic magnetization is significantly enhanced. Magneto optical imaging and study of the field dependence of the remanent magnetization as a function of particle size both show that global currents over the whole sample do exist but that the intergranular and intragranular current densities have distinctively different temperature dependences and differ in magnitude by about 1000. Assuming that the highest current density loops are restricted to circulation only within grains leads to values of ~5×10 6 A/cm 2 at 5 K and self field, while whole-sample current densities, though two orders of magnitude lower are 1000-10000 A/cm 2 , some two orders of magnitude higher than in random polycrystalline cuprates. We cannot yet be certain whether this large difference in global and intragrain current density is intrinsic to the oxypnictides or due to extrinsic barriers to current flow, because the samples contain significant second phase, some of which wets the grain boundaries and produces evidences of SNS proximity effect in the whole sample critical current. 2 Introduction:The recent discovery of superconductivity in the LaFeAsO 1-x F x compound [1] has stimulated a rapid exploration of superconductivity in the rare earth iron oxypnictides [2][3][4][5][6][7][8][9][10][11][12][13][14]. It has now been established that the iron oxypnictides can be superconducting when doped to x ~0.05-0.2 and that they can have transition temperature T c above 40 K when La is replaced by Ce [5] and above 50 K by Pr, Nd, Sm and Gd [7][8][9][10][11]. In a recent paper [12] we addressed the issue of electromagnetic granularity in polycrystalline La iron oxypnictides, finding an asymmetric M(H) loop that indicated an irreversible moment due to hysteretic bulk currents that was almost as small as the reversible magnetization of the superconducting state. In that case we were not able to distinguish definitively between a state where the intragrain pinning was very weak, leading to very low intragrain current densities or to the state where currents were largely confined to the intragrain regions and might have been rather high. Based on the rather high upper critical field B c2 (0) values of 63-65 T observed by Hunte et al.

Using x-ray absorption and resonant inelastic x-ray scattering, charge dynamics at and near the Fe L edges is investigated in Fe pnictide materials, and contrasted to that measured in other Fe compounds. It is shown that the XAS and RIXS spectra for 122 and 1111 Fe pnictides are each qualitatively similar to Fe metal. Cluster diagonalization, multiplet, and density-functional calculations show that Coulomb correlations are much smaller than in the cuprates, highlighting the role of Fe metallicity and strong covalency in these materials. Best agreement with experiment is obtained using Hubbard parameters U 2eV and J ≈ 0.8eV.

We use powder neutron diffraction to study the spin and lattice structures of polycrystalline samples of nonsuperconducting PrFeAsO and superconducting PrFeAsO 0.85 F 0.15 and PrFeAsO 0.85 . We find that PrFeAsO exhibits an abrupt structural phase transitions at 153 K, followed by static long range antiferromagnetic order at 127 K.Both the structural distortion and magnetic order are identical to other rare-earth oxypnictides. Electron-doping the system with either Fluorine or oxygen deficiency 2 suppresses the structural distortion and static long range antiferromagnetic order, therefore placing these materials into the same class of FeAs-based superconductors.

Here we report the fabrication and superconductivity of the iron-based arsenic-oxide GdFeAsO 1-δ compound with oxygen-deficiency, which has an onset resistivity transition temperature at 53.5 K. This material has a same crystal structure as the newly discovered high-T c ReFeAsO 1-d family (Re = rare earth metal) and a further reduced crystal lattice, while the T c starts to decrease compared with the SmFeAsO 1-δ system.