Imaging the coexistence of a superconducting phase and a charge-density modulation in the K0.73Fe1.67Se2superconductor using a scanning tunneling microscope

Abstract

We report scanning tunneling microscopy studies of the local structural and electronic properties of the iron selenide superconductor K${}_{0.73}$Fe${}_{1.67}$Se${}_{2}$ with ${T}_{\mathrm{C}}$ $=$ 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 $\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}$ 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.