Ricerc@Sapienza

Recently, a new family of iron-based superconductors, namely AeAFe4As4 (1144) with Tc ~36 K, has been discovered. The most studied system among '1144' type compounds is the CaKFe4As4 that shows ultrahigh critical current and hedgehog type of magnetic ordering. The critical density is highest at high magnetic field among known superconductors including the high Tc copper oxide superconductors. Here, we propose to perform a study of structure-function relationship in CaKFe4As4 using spectroscopy and microscopy by varying the chemical composition, annealing procedures , external temperature and pressure. The nanoscale structure will be studied by X-ray absorption spectroscopy while the electronic structure will be studied by photoemission and spectromicroscopy. Infrared and Raman spectroscopy will also be used to study optical properties. The studies are expected to be focussed on pristine CaKFe4As4 and differently annealed samples showing different critical currents. Another system of interest is EuRbFe4As4 exhibiting coexistence of superconductivity and ferromagnetism in addition of high critical current density. The unusual ferromagnetic-superconductor coexsistence, probably driven by Eu+2 ions, deserves particular attention and experimental efforts, as its exact nature is unknown. Possible texturing will be studied by spectromicroscopy and infrared microscopy. The proposed experiments have direct implications on our understanding of the intrinsic flux pinning mechanism for ultrahigh critical density in these materials.