Since the discovery that electrons in graphene behave as massless Dirac fermions (see Physics Today, January 2006, page 21), the single-atom-thick material has become a fertile playground for testing exotic predictions of quantum electrodynamics. Graphene experiments have been used successfully to probe such relativistic phenomena as Klein tunneling—the anomalous, unimpeded passage of electrons through a high potential barrier—and the fractional quantum Hall effect (see Physics Today, January 2010, page 11). Now add to that list atomic collapse, the spontaneous formation of electrons and positrons in the electrostatic field of a superheavy atomic nucleus. Although the phenomenon has eluded detection in collider experiments, its condensed-matter analogue was recently spotted in a collaborative effort headed by Michael Crommie (University of California, Berkeley) and Leonid Levitov (MIT).1
Atomic collapse is thought to effectively set a ceiling on the periodic table: For any atom whose proton number exceeds some...
