Correlated behavior, such as superconductivity, is more likely to arise in materials with a flat band structure—that is, many electrons or holes sharing the same energy. A flat band near the Fermi level, or the highest occupied energy states, boosts the many-body interactions required for such behavior. The typical Fermi level for graphene (blue plane in figure), however, is nowhere close to a flat band. For a single undoped layer, it consists of only isolated states, the Dirac points (black and white dots).
Graphene’s band structure does have a flat region, known as the van Hove singularity (red dot), but it is several electron volts above the Dirac points. Although introducing more charge carriers raises the Fermi level, doping the material enough to push the Fermi level to the van Hove singularity has proven challenging.
Now Ulrich Starke of the Max Planck Institute for Solid State Research in Stuttgart and his colleagues have doped graphene above the van Hove singularity. Their so-called overdoped graphene is predicted to host correlated states, such as chiral superconductivity, magnetism, or spin- or charge-density waves.
Reaching a charge-carrier density of up to 5.5 × 1014 cm−2 took two steps. First, the researchers intercalated graphene with ytterbium atoms, which wedged themselves between the graphene layer and the silicon carbide substrate and, once there, transferred charge carriers. Previous studies that used intercalation of other atoms, such as calcium, reported Fermi levels close to the van Hove singularity. Ytterbium intercalation lifted graphene’s Fermi level up to the singularity. Tuning the level any higher than that required the second step: potassium adsorption.
Starke and his colleagues, in collaboration with the Helmholtz Center Berlin for Materials and Energy, used angle-resolved photoelectron spectroscopy at a temperature of 20 K to take images of the band structure at different potassium doses. When the Fermi level was at the van Hove singularity, the researchers observed a flat band extending over much of momentum space. At higher doping, they found that the topology of the Fermi surface changed, a so-called Lifshitz transition. (The phenomenon is named for solid-state theorist Ilya Lifshitz; see the article by Alexander Grosberg, Bertrand Halperin, and John Singleton, Physics Today, November 2017, page 44.) The Fermi surface switched from two regions of filled (electron) states to one larger region of empty (hole) states. The transition confirmed that the graphene was doped beyond the van Hove singularity.
Future studies will test whether the new doping regime does indeed yield correlated states. Theorists have predicted that overdoped graphene may display, among other phases, chiral superconductivity—a form of topological superconductivity that breaks time-reversal symmetry and has been observed in superfluid helium-3 but not yet in solid-state systems. (P. Rosenzweig et al., Phys. Rev. Lett. 125, 176403, 2020; thumbnail illustration credit: MPI for Solid State Research/P. Rosenzweig et al.)
