Superconductivity in Transition Metal Substituted Iron-Based Superconductors

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I report on superconductivity in undoped SrFe2As2 and find that it is caused by lattice strain in the as-grown crystals that can be removed or returned with annealing or pressure, respectively. To study the magnetic/structural transition I measure the evolution of these transitions in solid solutions of the [Ca, Sr, Ba]Fe2As2 series and determine that the Neel temperature is independent of the size of the antiferromagnetically ordered moment. I present the first reported phase diagrams for Ni- and Pt-substitution in SrFe2As2, showing that the simple charge-counting picture of chemical substitution cannot completely describe the onset and offset of the superconducting phase. Finally, I use the transport scattering rate to explain the variation in Tc seen in transition metal substituted 122s. I will show that pair breaking can explain the variation in the optimum transition temperature, and that the rate of suppression of Tc with scattering will show that the pairing symmetry of the iron-based superconductors is a sign-changing, multiband s-wave order parameter that must include both inter- and intraband scattering.