Angle-resolved photoemission study of the graphite intercalation compound KC8 : A key to graphene

A. Grüneis, C. Attaccalite, A. Rubio, D. V. Vyalikh, S. L. Molodtsov, J. Fink, R. Follath, W. Eberhardt, B. Büchner, and T. Pichler, Phys. Rev. B 80, 075431 (2009)

Looking at massless electrons of graphene layers inside graphite

Every week there are numerous papers explaining the exotic features of graphene, a single layer of carbon atoms arranged in a honeycomb lattice. Due to the honeycomb lattice, electrons in graphene behave as massless particles. In ideal, freestanding graphene, electrons can move through the crystal without scattering with carbon atoms. This is very much the way that light propagates (with a velocity that is only 1/300 of the speed of light). Therefore electrons in graphene are described by the Dirac equation – indeed this is something very unique in condensed matter physics. Unfortunately this ideal view of graphene contrasts with experimental evidences where coupling with the substrate or to adjacent graphene layers spoils the massless Dirac Fermion behaviour predicted from the theory. In this case the substrate interaction causes the electronic structure to open up a gap and the electrons to acquire a finite mass. Then they are no longer described by the Dirac equation.

In two recent papers in Physical Review B, A. Grüneis and his coworkers studied graphene layers inside potassium doped graphite. The potassium ions move in between individual graphene sheets (this process is also known as intercalation) thereby separating adjacent graphene layers apart and cancelling their interaction. The authors could show that this material then retains the properties of Dirac Fermions again because it is composed of isolated, non-interacting graphene layers. Combining angle-resolved photoemission spectroscopy measured at the BESSY synchrotron in Berlin and theoretical calculations from the European Theoretical Spectroscopy Facility, they unravelled the full experimental Dirac cone of electrons in graphene revealing a linear dispersion in all directions, the same way how light propagates (see Figure). Moreover the potassium present in this material acts as a dopant, pumping even more electrons to graphene. This allowed them to measure not only the valence bands but also the conduction bands of graphene for a large energy range, not reachable in single layer graphene.