Intrinsic Superconducting Gap in Bilayer KCa ₂ Fe ₄ As ₄ F ₂ and Decoupled Monolayer FeAs

Despite extensive progress in iron-based superconductors, how interlayer coupling affects superconductivity remains a key unresolved issue. Here, using angle-resolved photoemission spectroscopy, we successfully resolve the electronic structure of a surface-decoupled FeAs monolayer in KCa 2 Fe 4 As 4 F 2, a rare example in iron pnictides. This allows for a direct, side-by-side comparison between the basic FeAs unit and its bulk bilayer counterpart within a single sample. Two distinct superconducting phases are identified: a bulk bilayer phase with Tc ∼ 34 K and a surface-decoupled monolayer phase with a smaller gap vanishing at ∼18 K. In addition to probable enhanced fluctuations in single-layer FeAs, we propose that this Tc difference may originate from either interfacial effect, such as phonon-mediated assistance from the CaF layers or electronic interlayer hopping within the bilayer unit. Our work offers a refined framework for understanding the superconductivity in multilayer iron-based superconductors.