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Observing Atomic Collapse Resonances in Artificial Nuclei on Graphene

Science  10 May 2013:
Vol. 340, Issue 6133, pp. 734-737
DOI: 10.1126/science.1234320

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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.

Abstract

Relativistic quantum mechanics predicts that when the charge of a superheavy atomic nucleus surpasses a certain threshold, the resulting strong Coulomb field causes an unusual atomic collapse state; this state exhibits an electron wave function component that falls toward the nucleus, as well as a positron component that escapes to infinity. In graphene, where charge carriers behave as massless relativistic particles, it has been predicted that highly charged impurities should exhibit resonances corresponding to these atomic collapse states. We have observed the formation of such resonances around artificial nuclei (clusters of charged calcium dimers) fabricated on gated graphene devices via atomic manipulation with a scanning tunneling microscope. The energy and spatial dependence of the atomic collapse state measured with scanning tunneling microscopy revealed unexpected behavior when occupied by electrons.

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