Observing Atomic Collapse Resonances in Artificial Nuclei on Graphene.

ROM 2013-6
Author: Yang Wang1,2 / Dillon Wong1,2, / Andrey V. Shytov3, / Victor W. Brar1,2,/ Sangkook Choi1, /Qiong Wu1,2, / Hsin-Zon Tsai1, / William Regan1,2, / Alex Zettl1,2, / Roland K. Kawakami5, / Steven G. Louie1,2, / Leonid S. Levitov4, / Michael F. Crommie1,2,†
Institute: 1 Department of Physics, University of California at Berkeley, Berkeley, CA 94720, USA. / 2 Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA. / 3 School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, UK. / 4 Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA. / 5 Department of Physics and Astronomy, University of California at Riverside, Riverside, CA 92521, USA.
Instrument: LT STM

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.