Graphene gives glimpse of atomic collapse

According to the Dirac equation, any atomic nucleus with a proton number exceeding some critical value—thought to be around 170—should produce a Coulomb force strong enough to spontaneously create an electron–positron pair from the vacuum. Known as atomic collapse, or the sparking of the vacuum, the effect has eluded experimental observation, mainly due to the difficulty of creating the requisite super heavy nuclei. Now Michael Crommie (University of California, Berkeley), Leonid Levitov (MIT), and coworkers have observed the phenomenon's condensed-matter analogue. In their experiment, calcium dimers—which acquire a charge +e when adsorbed atop single-layer graphene—play the role of protons; the five-dimer cluster shown in this scanning tunneling microscopy image is the researchers' equivalent of a boron nucleus. The graphene itself serves as the vacuum: Its electrons and holes, which behave like massless, relativistic particles, are analogous to the virtual particles that populate the vacuum in quantum field theories. Because the effective fine-structure constant, a measure of the relative strength of charge interactions, is large in graphene, the single-atom-thick material was predicted to yield atomic collapse at a critical charge closer to 1 than to 170. Indeed, on forming a cluster of just five calcium dimers, the team detected the smoking gun of atomic collapse—the sudden emergence of a bound electronic state, visible as the bright halo in the image. More than just a curiosity, the collapse state could shed light on fundamental questions regarding the stability of matter. (Y. Wang et al., Science, in press, doi: 10.1126/science.1234320.)—Ashley G. Smart