Anomalous Band Bending in the (K $${}_{\mathbf{0.8}}$$ Na $${}_{\mathbf{0.2}}$$ ) $${}_{\mathbf{0.8}}$$ Fe $${}_{\mathbf{1.8}}$$ Se $${}_{\mathbf{2}}$$ Superconductor

Abstract

Iron chalcogenides intercalated with alkali metal atoms attract the attention of physicists due to their unusual natural phase segregation, where superconducting clusters form at the boundaries of the antiferromagnetic phase. In this work, using photoelectron spectroscopy, we discovered an unusual effect that presumably arises due to this phase segregation. We studied temperature dependences of the photoelectron spectra of Se 3d, Fe 3p, and the valence band at temperatures above and below \(T_{c}\approx 27\) K of the compound (K \({}_{0.8}\) Na \({}_{0.2}\) ) \({}_{0.8}\) Fe \({}_{1.8}\) Se \({}_{2}\) with substitution of alkali metal atoms. A strong temperature dependence was found for both the valence band and the core levels: we observed a significant broadening of the spectra, which monotonically decreased with increasing sample temperature under cyclical temperature change. We believe that this broadening is associated with the appearance of volume charges in the dielectric matrix, leading to the band bending. Moreover, the shape of the potential that arises under the surface of this compound was restored, and an estimate was obtained for the relative amount of the superconducting phase. These results will help to better understand the physical processes occurring in this compound.