Anomalous band bending in the (K 0.8 Na 0.2) 0.8 Fe 1.8 Se 2 superconductor

Abstract

Iron dichalcogenides intercalated with alkali metal atoms attract the attention of physicists due to their unusual natural phase separation, in which superconducting clusters are formed 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 separation. We studied the temperature dependences of the photoelectron spectra of Se 3d, Fe 3p and the valence band at temperatures above and below Tc ≈ 27 K of the compound (K0.8Na0.2)0.8Fe1.8Se2 with a unique substitution of alkali metal atoms. A strong temperature dependence of both the valence band and core levels was discovered: we observed a strong broadening of the spectra, which monotonically decreased with increasing sample temperature, while the temperature changes were cyclic. We believe that this broadening is associated with the appearance of space charges in the dielectric matrix, which leads to band bending. Moreover, the shape of the potential arising under the surface of a given compound was reconstructed, and an estimate was obtained for the relative amount of the superconducting phase. The results obtained will help to better understand the physical processes occurring in this compound.