Electronic ground states and phase diagrams of Fe-pnictide superconductors

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The electronic phase diagrams of transition metal pnictides exibit a complex interplay of superconductivity and magnetism. Low moment magnetic spin density wave (SDW) ordering of the transition metal is found next to multi-band superconductivity characterized by a coexistence of different energy gaps on different Fermi surface sheets. In the presence of rare earth elements independent rare earth magnetic ordering is found as well as interacting rare earth – transition metal magnetically ordered phases. Using the local probes muon spin relaxation (μSR) and Mössbauer spectroscopy we determine the electronic ground states and the phase diagram in iron pnictides and chalgogenides with increasing interlayer distance, labeled 011, 122, 1111, and 22426. We determine independently the magnetic and superconducting order parameters and their respective volume fractions as a function of charge carrier doping, magnetic field and external pressure. Of particular interest in the second funding period are systems where a microscopic coexistence of superconductivity and magnetic order is found. In doped 122 Fe pnictides a direct interplay of the respective order parameters is found. This interplay shall be studied in detail to reveal the underlying microscopic mechanisms. In the iron-based superconductors several different substitution strategies lead to superconductivity, i.e. substituting mono or trivalent elements on the A site or different transition metals on the Fe-site in the electronically active Fe pnictide or chalgogenide planes. Besides charge carrier doping mechanisms also the scattering potentials are very different. It shall be investigated how this influences the properties of the electronic ground state phases. In 1111 systems we plan to examine new samples prepared by high pressure synthesis for which a better homogeneity and the possibility to grow single crystals is anticipated.