Comment on ‘‘Structure of the Mn-induced Cu(100)c(2×2) surface’’

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

“…This interpretation is in accordance with the conventional understanding of STM images of metal surfaces, which is based on the concept that electrons screen the positive charge of the nuclei, and the STM tip follows the density distribution of the surface atoms. It was confirmed by an STM experiment 22 that a single Mn impurity in the Cu͑100͒ surface is indeed imaged as a protrusion. Although the above-noted experimental findings lead to a consistent structural model of an ordered MnCu surface alloy, the estimated outward buckling of Mn varies significantly among the experiments.…”

“…Presumably, not the surface relaxation was directly observed, but the electronic contribution to the corrugation amplitude or LDOS, respectively, was the origin of the experimental results. The assumption that the corrugation amplitude measured in an STM experiment can be related to the surface relaxation, as it is underlying in the work of Noh et al, 20 van der Kraan and van Kempen, 21 and Wuttig et al, 22 is unjustified from our point of view. Therefore, it is most likely impossible to deduce the surface buckling from these STM-based experiments.…”

“…Meanwhile, a significant number of additional c(2ϫ2) MnX/X surface alloys have been found, i.e., XϭCu(110), 13 Co͑100͒, 14 Ni͑100͒, 15 Pd͑100͒, 16 and Ag͑100͒. [17][18][19] A recent set of scanning tunneling microscopy ͑STM͒ studies [20][21][22] confirmed the existence of a c(2ϫ2) surface unit cell, although only one chemical component of the surface alloy was imaged ͑for a typical STM image, see Fig. 5͒.…”

“…In the paper of Wuttig et al 22 a single Mn atom substituting a Cu atom in the Cu͑100͒ surface was used to measure the buckling of a Mn atom by STM. Assuming that the buckling of a single Mn impurity and that of Mn atoms in the c(2ϫ2) surface alloy is approximately the same, the STM experiment would give a direct estimate of the buckling of Mn in the alloy.…”

“…Although the above-noted experimental findings lead to a consistent structural model of an ordered MnCu surface alloy, the estimated outward buckling of Mn varies significantly among the experiments. [20][21][22] In this paper we analyze the STM images of the c(2 ϫ2) MnCu/Cu͑100͒ surface alloy on the basis of the model of Tersoff and Hamann 23 and first-principles electronic structure calculations. We show, while indeed a single Mn impurity is imaged as a protrusion, for a c(2ϫ2) MnCu/Cu͑100͒ surface alloy the Cu atoms and not the Mn atoms are imaged.…”