Atoms intercalated between sheets of graphene.

Graphite was first known as 'black lead', and the resemblance doesn't stop there - both lead and graphite are superconductors. Thomas Weller and co-authors have intercalated ytterbium or calcium atoms between graphene sheets and discovered superconductivity in these materials below 6.5 K and 11.5 K, respectively. Strangely, pushing the graphene layers further apart with these intercalant atoms makes the system more electronically isotropic. This work has generated a lot of theoretical activity, such as that by Gábor Csányi and collaborators. Their electronic structure calculations show that the electrons introduced into graphite through Yb and Ca doping do not behave conventionally. Instead of the graphene layers, they seem to prefer the void between the sheets. This would explain the increased isotropy, for example.

Letter by Weller et al. | Letter by Csányi et al. | News and Views by Klapwijk