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Giacomo Prando - Dresden, Allemagne

Electronic phase diagrams of 1111 oxy-pnictides investigated by μ +SR

Fe-based pnictides have attracted a remarkable attention in the latest years due to the appearance of high-Tc unconventional superconductivity under specific chemical and/or physical circumstances, i. e., suitable chemical substitutions and/or external pressure P. In this talk, I review how the generic electronic phase diagram can be related to what observed for, e. g., heavy fermions and cuprates. The issue of the nanoscopic coexistence between magnetism and superconductivity is addressed with specific reference to results of mu+SR in RFeAsO oxy-pnictide materials (R : rare-earth ion). Out-of-plane O1-xFx and in-plane Fe1-xCox substitutions are discussed in the light of the typical interpretation in terms of charge doping. The effect of other impurities such as Ru and Mn on the overall phase diagram is addressed as well. At the same time, local distortions and the induced modification in the lattice parameters play a key role as well in the physical properties of RFeAsO oxy-pnictides. In order to deepen this issue, I discuss how P affects the electronic properties of 1111 materials, again from the point of view of mu+SR. In the limit of low values of chemical substitutions, the P-induced suppression of the spin-density-wave phase is presented. The region of coexistence between magnetism and superconductivity is further addressed, where P is exploited to study narrow regions of the electronic phase diagrams too difficult to explore finely by means of a chemical control parameter. Finally, recent results are presented dealing with the dependence of the superfluid density on P in optimally-doped LaFeAsO1-xFx. Then, I move to the opposite side of the phase diagram, obtained by the Fe1-xCox substitution for x = 1. RCoAsO materials are itinerant ferromagnets and their behaviour under P is discussed side by side to what was found for the isostructural RCoPO after a systematic study for different R ions. Insights are provided about the enhancement of the ferromagnetic phase and, in particular, computational results are presented dealing with the strong analogies detected among chemical and external pressures.