Imaging the coexistence of a superconducting phase and a charge-density modulation in the K0.73Fe1.67Se2 superconductor using a scanning tunneling microscope

We report scanning tunneling microscopy studies of the local structural and electronic properties of the iron selenide superconductor K0.73Fe1.67Se2 with T-C = 32 K. On the atomically resolved FeSe surface, we observe a well-defined superconducting gap and the microscopic coexistence of a charge-density modulation with root 2 x root 2 periodicity with respect to the original Se lattice. We propose that a possible origin of the pattern is the electronic superstructure caused by the block-antiferromagnetic ordering of the iron moments. The widely expected iron vacancy ordering is not observed, indicating that it is not a necessary ingredient for superconductivity in the intercalated iron selenides.