Heterogeneity in ultrathin films simulated by Monte Carlo method*

The 3D composition profile of ultra-thin Pd films on Cu(001) has been experimentally determined using low energy electron microscopy (LEEM).$^{[1]}$ Quantitative measurements of the alloy concentration profile near steps show that the Pd distribution in the 3$^{rd}$ layer is heterogeneous due to step overgrowth during Pd deposition. Interestingly, the Pd distribution in the 2$^{nd}$ layer is also heterogeneous, and appears to be correlated with the distribution in the 1$^{st}$ layer. We describe Monte Carlo simulations that show that correlation is due to Cu-Pd attraction, and that the 2$^{nd}$ layer Pd is, in fact, laterally equilibrated. By comparing measured and simulated concentration profiles, we can estimate this attraction within a simple bond counting model. [1] J. B. Hannon, J. Sun, K. Pohl, G. L. Kellogg, \textit{Phys. Rev. Lett.} \textbf{96}, 246103 (2006)