Interpreting STM images of the MnCu/Cu(100) surface alloy

c(2 X 2)MnCu/Cu(100) is an ordered two-dimensional surface alloy that exhib its a checkerboard arrangement of Mn and Cu atoms on the Cu(100) surface. M n buckles outwards by 0.3 Angstrom with respect to Cv and in all previous s canning tunneling microscopy (STM) experiments only one chemical species wa s imaged which was assumed to be Mn. We analyze the STM results by first-pr inciples calculations based on the density-functional theory and show that Cu rather than Mn is imaged, while indeed Mn is imaged as single Mn impurit ies at Cu(100). We explain this result in terms of the formation of Mn stat es bridging over the Cu atoms. These Mn states are characteristic for Mn in a c(2X2)MnCu surface alloy. Missing Mn atoms break this bridging bond and the surrounding Cu atoms are imaged as depressions.