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.
The research proposed in the project is to quantify physical parameters related with the nanoscale structure, electronic structure and phonons in newly discovered 1144-type iron-based superconductor family which has large potential in practical applications due to ultrahigh critical current density. In particular, we have proposed to study CaKFe4As4 and EuRbFe4As4 systems showing superconductivity at ~36 K and high critical current with the latter being characterized by coexisting superconductivity and magnetism. The cause of ultrahigh critical current is still unknown and it is expected that the flux-pinning being intrinsic to the system. Since flux pinning is expected to be related with the nanoscale structure, therefore, a systematic study is required to quantify the local disorder and electronic properties. Moreover, optimizing pinning properties is expected to help in developing new systems with desired applications. For the purpose, we will use different spectroscopic and microscopic techniques with the team composed by physicists and materials scientists. For the experimental part, spectroscopy techniques namely x-ray absorption is to be used to probe site selective local structure and valence electronic structure. Photoemission and infrared spectroscopy are to be used to determine parameters of the electronic structure, while the information on the phonon structure by Raman and Infrared spectroscopy. The spectroscopy will be combined with photoelectron microscopy and infrared microscopy together with atomic force microscopy.
The required samples are to be prepared in AIST Tsukuba (where this new family of superconductors was discovered) and Okayama University in Japan and the coordinator of this project (N.L. Saini) is already collaborating with the two institutes. In particular, there is bilateral agreement between Okayama University and Sapienza with the coordinator being the responsible of this agreement. Under this agreement, there are young researchers from Okayama University has been frequent visitor of Sapienza. The project participants have proven record of their expertise, making the proposed objectives realizable. The proposed research is expected to quantify physical parameters which are related with the intrinsic pinning mechanism and hence the ultrahigh critical current density with a final goal to develop materials with desired physical properties for practical applications. Therefore the the proposed research has large impact on the potential use of practical applications of these superconductors.