Of all the unique properties of strength, flexibility and intriguing optoelectronic behavior, one phenomenon is notably absent from the repertoire of graphene: superconductivity. If graphene could be doped up to becoming a superconductor a wealth of new efficiencies of applications such as nanoscale superconducting quantum interference devices, single-electron superconductor-quantum dot devices, nanometre-scale superconducting transistors and cryogenic solid-state coolers would emerge.
European researchers report in Nature Physics how alkali metal adatoms, and specifically lithium atoms, could make a graphene superconductor a reality. They hoped that they might be able to emulate the success seen with graphite intercalated compounds containing lithium [Profeta et al., Nature Phys (2012) doi: 10.1038/nphys2181]. They have now demonstrated superconductivity with lithium-covered graphene and found it to become functional at a much higher temperature than the calcium analogue. The team explains that the superconductivity arises through a phonon-mediated enhancement of electron-phonon coupling in the doped material.
Initially, the researchers had thought that superconductivity might be induced in graphene simply by filling the carbon pi-states using rigid-band doping to endow it with sufficient charger carriers to let the current flow freely and unhindered. However, not only is that process slow, the resulting material would not be superconducting because symmetry would forbid the necessary coupling between pi-states and vibrational states out-of-plane.
The team realized that to facilitate this essential coupling process and so make graphene superconducting they would have to promote new electronic states of the Fermi kind as occurs in the intercalated graphite superconductors. In those materials carrier numbers are raised sufficiently, coupling to carbon out-of-plane vibrations is promoted and the appropriate coupling to intercalant vibrations occurs.
The suspicion was that calcium would be the most effective dopant for graphene, given that it produces the highest critical temperature in the graphite intercalates. This notion was not supported by their calculations for a calcium layer on graphene, which showed that the critical temperature would be lower. Lithium, on the other hand, might be different. Although the intercalated lithium-graphite material is not superconducting, the calculations on lithium-coated graphene offered a different view. A single layer of lithium would no longer be confined in the quantum step and could step up to the Fermi level.
Indeed, further calculations confirmed that this might be the case and experiments at close to absolute zero verified that the lithium-coated graphene has a superconducting critical temperature of up to 8.1 K. Thus superconductivity can now be added to graphene's repertoire.