Interpreting STM images of the MnCuÕCu„100... surface alloy

c(232)MnCu/Cu(100) is an ordered two-dimensional surface alloy that exhibits a checkerboard arrangement of Mn and Cu atoms on the Cu~100! surface. Mn buckles outwards by 0.3 Å with respect to Cu and in all previous scanning tunneling microscopy ~STM! experiments only one chemical species was imaged which was assumed to be Mn. We analyze the STM results by first-principles calculations based on the densityfunctional theory and show that Cu rather than Mn is imaged, while indeed Mn is imaged as single Mn impurities at Cu~100!. We explain this result in terms of the formation of Mn states bridging over the Cu atoms. These Mn states are characteristic for Mn in a c(232)MnCu surface alloy. Missing Mn atoms break this bridging bond and the surrounding Cu atoms are imaged as depressions.