Dopant ordering and enhancement of superconducting transition temperature induced by co-doping of iron-based superconductors

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Iron-based superconductors consist of alternating two-dimensional Fe2As2layers and spacer layers. In order to realize higher transition temperature Tc, it is essential to seek novel spacer layers to optimize the electronic state in Fe2As2layer. The spacer layers reported so far consist of ionic chemical bonds. In this study, we discovered iron-based superconductors Ca10(PtnAs8)(Fe2-xPtxAs2)5(n = 3, 4) with PtnAs8spacer layers made of covalent chemical bonds. Tc= 17 K was observed in the beta phase (n = 3), in which Pt and vacancy are ordered in the spacer layer. By adjusting the amounts of Pt and vacancy, Tcwas raised up to 38 K in the alpha phase (n = 4). Our results demonstrated that a variety of spacer layers can be included in the iron-based superconductors.