Microstructure Morphology of Chemical and Structural Phase Separation in Thermally Treated KFeSe Superconductor.

The iron-based KFeSe superconductor displays chemical, structural and electronic phase separation at nanoscale to microscale, leading to the coexisting metallic phase embedded in an antiferromagnetic host matrix. The metallic character of the system is believed to arise from a percolative granular network affecting its transport in the normal as well as in the superconducting state. This percolative network can be manipulated and controlled through thermal treatments. In this study, we have used scanning X-ray micro-fluorescence to visualize morphology of the chemical phase separation coupled to the percolation in KFeSe, manipulated by two distinct thermal treatments, i. e., fast quenching and slow cooling. We find a differing spatial correlation between Fe and K distributions in the two samples, ascribed to a different degree of Fe vacancy ordering. We have also identified an intermediate phase that acts as an interface between the two main phases. The high temperature quenching produces directionally oriented clustered microstructure in which the percolation threshold is lower and hence a more effective transport networks. Instead, the slow cooling results in larger interfaces around the percolation threshold that seems to affect the superconducting properties of the system. The results provide a quantitative characterization of microstructural morphology of differently grown KFeSe showing potential for the design of electronic devices based on sub-micron scale chemical phase separation, thus opening avenues for further studies of complex heterogeneous functional structures.