Physical origin of exchange diffusion on fcc(100) metal surfaces

For the (100) surfaces of Pt and Ir, experiments revealed that self-diffusion proceeds by atomic exchange rather than hopping. Using density-functional theory we find that the physical origin of this phenomenon is different from that for Al, where it had been explained as being activated by the formation of directional bonds at the transition state. We predict that exchange diffusion should also occur on Au(100) and show that the tensile surface stress plays a crucial role for the exchange diffusion. This explains why the exchange process is favorable for the late $5d$, but not for $3d$ and $4d$ fcc(100) metal surfaces.