Thermodynamic separability of ultra-thin film surfaces and interfaces

Surface and interface excess quantities are coupled or ill-defined for ultra-thin films. Yet, surface and interface energy are consistently used to describe monolayer-range heteroepitaxial structures. Here, we examine the convergence of interfacial thermodynamic quantities to the well-defined thermodynamic (separability) limit in the systems Ag/Au(1 1 1) and Al/Au(1 1 1). Convergence to separability occurs together with the interface-induced electronic structure and interlayer spacing perturbations. We discuss how lack of thermodynamic separability affects growth mode, coherency, and domain organization.