Dancing Loops: How Interstitial Clusters Move in Irradiated Nickel
A nickel crystal irradiated with ions resembles a hall where extra chairs are left after a concert. Those chairs are atoms squeezed between lattice sites. They first gather into clusters, then arrange into rings: some rings lie flat on the floor, immobile, while others roll across the hall like hoops. The question of why some roll and others do not has long troubled materials scientists.