Role of stress in thin film alloy thermodynamics: Competition between alloying and dislocation formation

Using scanning tunneling microscopy (STM) and first-principles local-spin-d snsity-approximation calculations to stud; submonolayer films of Co1-cAgc/R u(0001) alloys, we have discovered a novel phase-separation mechanism. When the Ag concentration c exceeds 0.4, the surface phase separates between a dislocated, pure Ag phase and a pseudomorphically strained Co0.6Ag0.4 surfa ce alloy. We attribute the phase separation to the competition between two stress relief mechanisms: surface alloying and dislocation formation. The a greement between STM measurements and our calculated phase diagram supports this interpretation.