Structural investigations on the K0.8Fe1.6+x Se2 superconducting materials have revealed microstripes arising from phase separation. Two coexisting structural phases can be characterized by modulations of q 1 = 1/5[3a* + b*], the antiferromagnetic phase K0.8Fe1.6Se2, and q 2 = 1/2[a* + b*], the superconducting phase K y Fe2Se2, respectively. These stripe patterns also show visible correlation with the anisotropic assembly of superconducting particles along the [110] and [11̅0] directions. In addition to the notable stripe structures, a nanoscale phase separation also appears in the present superconducting system as clearly observed by high-resolution transmission electron microscopy. Certain nanoscale structural features in this heterogeneous system can be qualitatively understood by the percolation scenario.

We report an in-plane optical spectroscopy study on the iron-selenide superconductor K0.75Fe1.75Se2. The measurement revealed the development of a sharp reflectance edge below Tc at frequency much smaller than the superconducting energy gap on a relatively incoherent electronic background, a phenomenon which was not seen in any other Fe-based superconductors so far investigated. Furthermore, the feature could be noticeably suppressed and shifted to lower frequency by a moderate magnetic field. Our analysis indicates that this edge structure arises from the development of a Josephson-coupling plasmon in the superconducting condensate. Together with the transmission electron microscopy analysis, our study yields compelling evidence for the presence of nanoscale phase separation between superconductivity and magnetism. The results also enable us to understand various seemingly controversial experimental data probed from different techniques.