Common (π,π) Band Folding and Surface Reconstruction in FeAs-Based Superconductors

High resolution angle-resolved photoemission spectroscopy (ARPES) measurements are carried out on CaKFe 4 As 4 , KCa 2 Fe 4 As 4 F 2 and (Ba 0.6 K 0.4 )Fe 2 As 2 superconductors. Clear evidence of band folding between the Brillouin zone center and corners with a ( π , π ) wave vector has been found from the measured Fermi surface and band structures in all the three kinds of superconductors. A dominant 2 × 2 surface reconstruction is observed on the cleaved surface of CaKFe 4 As 4 by scanning tunneling microscopy (STM) measurements. We propose that the commonly observed 2 × 2 reconstruction in the FeAs-based superconductors provides a general scenario to understand the origin of the ( π , π ) band folding. Our observations provide new insights in understanding the electronic structure and superconductivity mechanism in iron-based superconductors.