Macroscopic phase segregation in superconducting K_0.73Fe_1.67Se₂ as seen by muon spin rotation and infrared spectroscopy

Using muon spin rotation and infrared spectroscopy, we investigated the recently discovered superconductor K0.73Fe1.67Se₂ with Tc≈32 K. We show that the combined data can be consistently described in terms of a macroscopically phase-segregated state with a matrix of ∼88% volume fraction that is insulating and strongly magnetic and inclusions with an ∼12% volume fraction, which are metallic, superconducting, and nonmagnetic. The electronic properties of the latter, in terms of the normal state plasma frequency and the superconducting condensate density, appear to be similar as in other iron selenide or arsenide superconductors