Imaging the coexistence of superconductivity and a charge density modulation in K$_{0.73}$Fe$_{1.67}$Se$_{2}$ superconductor

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 $T_{C}$ = 32K. On the atomically resolved FeSe surface, we observe well-defined superconducting gap and the microscopic coexistence of a charge density modulation with $\sqrt 2 \times \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.