Observing Atomic Collapse Resonances in Artificial Nuclei on Graphene

Creating Unstable Atomic Orbitals A hallmark of atomic Bohr orbitals is that they are stable; that is, time independent. However, for a very highly charged nucleus, the electrons must be described with the relativistic Dirac equation; the motion becomes time dependent, with electrons spiraling into the nucleus and coupling to positrons at large distances from the nucleus. In graphene, charge carriers are mass-less and described by the relativistic Dirac equation, and could also exhibit “atomic collapse” states. Wang et al. (p. 734 , published online 7 March) created highly charged clusters of calcium dimers by atomic manipulation with a scanning tunneling microscope. The emergence of atomic-collapse resonances with increasing cluster size and charge was observed with scanning tunneling microscopy.