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1We present a detailed study of the intrinsic electronic phase diagram of the oxypnictide superconductors in the normal state based on the analysis of the electrical resistivity ρ of both LaO1-xFxFeAs and SmO1-xFxFeAs for a wide range of doping. Our data give clear-cut evidence for unusual normal state properties in these new materials. As a function of doping ρ of LaO1-xFxFeAs shows a clear transition from pseudogap to Fermi liquid-like behavior, mimicking the phase diagram of the cuprates. Moreover, our data reveal a correlation between the strength of the pseudogap signatures and the stability of the superconducting phase. The pseudogap signatures, which are clearly connected with the structural and magnetic transitions of the parent material, become stronger in SmO1-xFxFeAs where superconductivity is enhanced and vanish when superconductivity is reduced in the doping region with Fermi liquid-like behavior [1]. We further present evidence for the connection between the pseudogap signatures in electrical transport and the slowing-down of spin fluctuation. 2Based on our recent magnetization, specific heat, resistivity, thermal expansion and magnetostriction studies on the family of iron-oxypinctide REFeAsO1-xFx (RE = La, Ce, Gd, Pr, Sm) we discuss the phase diagram with a particular focus on the evolution of magnetism in the normal and superconducting state. The undoped La-compound has a commensurate antiferromagnetically ordered ground state with a strongly reduced ordered moment. Strong structural fluctuations far above the orthorhombic phase transition exhibit positive pressure dependence and thus are not related to the magnetic ordering transition. In contrast, resistivity data indicate their intimate coupling to electronic degrees of freedom. Charge carrier doping improves the conductivity and suppresses magnetic order. Remarkably, pronounced antiferromagnetic interactions are present in the metallic normal state of the superconducting samples which underlines the relevance of magnetism. Unusual effects are also found if La is replaced by magnetic 4f ions. The magnetic ordering temperatures for the undoped compounds are nearly unchanged which indicates only weak steric effects. We find, however, strong magnetoelastic effects while the specific heat is magnetic field dependent. The particular thermodynamic signature of the superconducting transition in GdFeAsO1-xFx is discussed. |
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