  We report scanning tunneling microscopy studies of the local structural and
electronic properties of the iron selenide superconductor K0.73Fe1.67Se2 with
TC = 32K. On the atomically resolved FeSe surface, we observe well-defined
superconducting gap and the microscopic coexistence of a charge density
modulation with root2*root2 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.
