Evidence for saddle point driven charge density waves on the surface of heavily hole doped iron-based superconductor

When a band (a saddle point) is located just below the Fermi level (E_F) and is related to another saddle point via a wave vector q, Rice and Scott demonstrated that the wave vector dependent electron susceptibility χ(q) diverges logarithmically. A superlattice instability which does not have "nesting" in the usual sense can arise in the model. However, evidence for charge density wave (CDW) driven by saddle point has so far been elusive. Two-dimensional saddle points located near the Fermi level and unaffected by other energy bands have rarely been observed in previous materials. Here we use scanning tunneling microscope to demonstrate that 2×2 charge modulation along As-As direction with 2a_As period exists at the As-termination in heavily hole-doped 122 iron arsenide superconductors Ba_1-xK_xFe₂As₂ (x=0.77). An energy gap opens at the Fermi level and an intensity reversal exists in real space across the zero energy. A sharp peak in the tunneling spectra and theoretical calculation demonstrate the existence of saddle points in the middle of the principal axis of the first Brillouin zone. The vector of the 2×2 charge modulation connects the saddle points near the Fermi level, which gives strong evidence for the static charge ordering driven by saddle points. Our finding provides an intriguing platform for studying a new type of charge density wave on the arsenic (As) surface of heavily hole-doped 122 iron arsenide superconductors. Furthermore, it paves the way for exploring the intertwining between the unconventional superconducting pairing which breaks time reversal symmetry and the charge density wave.