Compact surface-cluster diffusion by concerted rotation and translation

First-principles calculations show that a pseudomorphic Ir monolayer on Lr( 111) has an unexpected metastable on-top configuration only slightly above the stable fee configuration. The energy required for any translation of th e monolayer parallel to the surface is small. This :opens the possibility t hat whole clusters can move by rotation and/or translation through configur ations: with many atoms in on-top sites. One such diffusion mechanism, the ''cartwheel shuffle'' could explain otherwise mysterious long jumps and hig h prefactors seen in the diffusion of 19-atom Ir clusters on Ir(111).