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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.

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