Observing Atomic Collapse Resonances in Artificial Nuclei on Graphene

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Science  07 Mar 2013:
DOI: 10.1126/science.1234320

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Relativistic quantum mechanics predicts that when the charge of a super-heavy atomic nucleus surpasses a certain threshold, the resulting strong Coulomb field causes an unusual "atomic collapse" state which 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 (STM). The energy and spatial dependence of the atomic collapse state measured using STM revealed unexpected behavior when it is occupied by electrons.

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