Visualization of the electronic phase separation in superconducting K_xFe_2−ySe₂

Type-II iron-based superconductors (Fe-SCs), the alkali-metal-intercalated iron selenide A_xFe_2−ySe₂ (A = K, Tl, Rb, etc.) with a superconducting transition temperature of 32 K, exhibit unique properties such as high Néel temperature, Fe-vacancies ordering, antiferromagnetically ordered insulating state in the phase diagram, and mesoscopic phase separation in the superconducting materials. In particular, the electronic and structural phase separation in these systems has attracted intensive attention since it provides a platform to unveil the insulating parent phase of type-II Fe-SCs that mimics the Mott parent phase in cuprates. In this work, we use spatial- and angle-resolved photoemission spectroscopy to study the electronic structure of superconducting K_xFe_2−ySe₂. We observe clear electronic phase separation of K_xFe_2−ySe₂ into metallic islands and insulating matrix, showing different K and Fe concentrations. While the metallic islands show strongly dispersive bands near the Fermi level, the insulating phase shows an energy gap up to 700 meV and a nearly flat band around 700 meV below the Fermi energy, consistent with previous experimental and theoretical results on the superconducting K_1−xFe₂Se₂ (122 phase) and Fe-vacancy ordered K_0.8Fe_1.6Se₂ (245 phase), respectively. Our results not only provide important insights into the mysterious composition of phase-separated superconducting and insulating phases of K_xFe_2−ySe₂, but also present their intrinsic electronic structures, which will shed light on the comprehension of the unique physics in type-II Fe-SCs.