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

High resolution angle-resolved photoemission spectroscopy (ARPES) measurements are carried out on CaKFe_4As_4, KCa_2Fe_4As_4F_2 and (Ba_0.6K_0.4)Fe_2As_2 superconductors. Clear evidence of band folding between the Brillouin zone center and corners with a (\pi,\pi) wave vector has been found from the measured Fermi surface and band structures in all the three kinds of superconductors. A dominant \sqrt2 \times \sqrt2 surface reconstruction is observed on the cleaved surface of CaKFe_4As_4 by scanning tunneling microscopy (STM) measurements. We propose that the commonly observed \sqrt2 \times \sqrt2 reconstruction in the FeAs-based superconductors provides a general scenario to understand the origin of the (\pi,\pi) band folding. Our observations provide new insights in understanding the electronic structure and superconductivity mechanism in iron-based superconductors.