In a “normal” atom, electrons occupy Bohr orbitals, which can be thought of as stable circular orbits around the nucleus. Many years ago, however, researchers predicted that if the charge of the atomic nucleus was increased to a very high value (above some threshold), then the electrons would no longer travel around in circular paths but instead would spiral in towards the nucleus and then out again. Such electrons have the possibility of escaping and flying away from the nucleus, and this behaviour is called “atomic collapse”.

The atomic collapse state is important for several reasons. First, it has been predicted by relativistic quantum mechanics, so actually seeing it in an experiment should help scientists better understand this basic theory. Second, atomic collapse tells us what would happen if we were to create atoms with very big nuclei (something that has not actually been done yet). Lastly, the collapse state provides precise information about how electrons in graphene will behave at very small length scales (of around 10 nm) near highly concentrated electrostatic charge. This information will be crucial for making nanodevices in which charged dopants and/or highly localized gate electrodes might be used to control electronic behaviour in graphene.

Artificial nuclei

The team, led by Mike Crommie of the University of California at Berkeley, was able to observe atomic collapse in graphene by building artificial supercritical nuclei “by hand” using a scanning tunnelling microscope (STM) tip. The researchers did this by sprinkling calcium atoms on top of a graphene field-effect transistor and then slightly warming the device to make the calcium atoms at the surface pair up, or form dimers, which are easier to manipulate with the STM tip than are single atoms. Next, they pushed the dimers into small clusters that behaved like artificial nuclei to all intents and purposes.

“Each charge calcium dimer in such a cluster plays the same role that a proton plays in a normal atomic nucleus,” explained Crommie. “Using our STM, we were thus able to directly image how electrons behave around a nucleus as we methodically increase the nuclear charge from below a supercritical limit (where there is no atomic collapse) to above a limit (where atomic collapse does occur).”

The signature of atomic collapse in our measurements is a special electronic state that we see at a particular energy, called the Dirac point, he told nanotechweb.org. “The atomic collapse state has spatial and energetic characteristics that are predicted by relativistic quantum mechanics, and our experimental observations match these predictions quite well.”

Miniature graphene devices

The work will be important for making future graphene devices that have been miniaturized down to length scales of around 10 nm, where atomic collapse characteristics become most important. In such devices, a single defect or charge dopant could have a huge effect on device behaviour and these defects and dopants might even be intentionally positioned in a device for particular applications.

“If such a dopant is highly charged (that is, if it is in the supercritical limit where atomic collapse occurs), then the electronic behaviour of the device will depend on the physics of the atomic collapse state,” said Crommie. “The same applies to electrostatic gate electrodes that have been shrunk down to below 10 nm length scales and placed near graphene. For some configurations (such as those with quantum dot-like geometries involving large potentials), the electronic behaviour of graphene should depend on atomic collapse physics.”

The team, which includes researchers from the Lawrence Berkeley National Laboratory, the University of Exeter in the UK, Massachusetts Institute of Technology and the University of California at Riverside, is now looking at how multiple interacting atomic collapse defects behave. “In other words, what happens when you take several atomic collapse ‘nuclei’ and place them near each other, so that they begin to overlap?” asks Crommie. “We are also very interested in understanding the effect of electron-electron interactions when you populate the atomic collapse state with more than one electron.”

The current work is published in Science.