Overdoping Graphene beyond the van Hove Singularity
Researchers from the Max Planck Institute for Solid State Research in Stuttgart, together with colleagues at the Helmholtz-Zentrum Berlin, have electron-doped a single layer of graphene up to unprecedented levels. The results open up a new and unexplored landscape in the phase diagram of graphene and revive the quest for correlated ground states such as chiral superconductivity or spin- and charge-density-wave order in this prototype two-dimensional material.
For more than ten years, theory has considered highly doped graphene as a promising candidate for the realization of exotic phases like chiral superconductivity or density-wave order. All of these exotic states are associated with the van Hove singularity, a particular level in the energy band structure of graphene that can accommodate an exceptionally large number of electrons. In this scenario, strong many-body interactions come into play, expected to push the system towards a variety of possible ordered ground states. However, experiment so far had great difficulty in keeping up with the predictions made by theory: doping up to the van Hove singularity remains very challenging, as it requires the transfer of more than 1014 electrons per cm2 onto the graphene layer.
In their most recent study published in Physical Review Letters, researchers in the Interface Analysis Group of the Max Planck Institute for Solid State Research in Stuttgart (MPI-FKF) have doped graphene even beyond its van Hove singularity – an important proof of principle that opens up a completely new regime in the phase diagram of this exciting material. The synchrotron-based experiments were carried out at BESSY II, Helmholtz-Zentrum Berlin, employing angle-resolved photoelectron spectroscopy to directly probe characteristic changes in the energy band structure of graphene upon overdoping.
When sandwiching the rare-earth element ytterbium in between the graphene layer and the supporting silicon carbide substrate, ytterbium donates electrons to graphene which is then pinned at the van Hove singularity in a first step. The advantage of this intercalation technique is that the surface of graphene remains free to occupy additional dopant atoms. In a second step, by depositing potassium atoms on top, the group were able to increase the carrier density by another 50%, thus doping graphene above the singularity level. "You could draw an analogy between overdoping and lifting a bulky object up the stairs to the top floor: it might only become possible by pushing from below (ytterbium) and simultaneously pulling from the top (potassium)", explains Philipp Rosenzweig, the lead author of the publication and a PhD student in the Scientific Facility for Interface Analysis at MPI-FKF.
Taking into account that the phase diagram of graphene around the van Hove singularity should be quite sensitive on the precise charge carrier density, ytterbium-intercalated graphene turns out to be even more versatile. Not only does it provide doping levels right at and above the singularity level, but also slightly below as demonstrated by the same group in an earlier publication. Ulrich Starke, who heads the Scientific Facility for Interface Analysis at MPI-FKF, therefore seems quite optimistic: "Now that we can routinely tune the doping level around van Hove filling in experiment, we have optimal conditions to hunt for correlated phases in graphene".