Temperature Dependence of the Local Structure and Iron Magnetic Moment in the Self-Doped CaKFe4As4 Iron-Based Superconductor

Temperature-dependent Fe K-edge extended X-ray absorption fine structure (EXAFS) and Fe Kβ X-ray emission spectroscopy (XES) measurements are used to study the local structure and local Fe magnetic moment in the self-doped CaKFe4As4 superconductor. The local Fe-As and Fe-Fe bond lengths show thermal expansion similar to the one found in other iron-based superconductors. The arsenic height from the Fe-Fe plane changes anomalously with a sharp decrease in the vicinity of the superconducting transition temperature. The Fe Kβ XES reveals that Fe in CaKFe4As4 is in the low spin state, and the local magnetic moment decreases sharply while the sample is cooled across the transition temperature. The results indicate importance of the magnetoelastic coupling in the superconductivity with a key role of interlayer atomic correlations in these materials.