Since the discovery of graphene more than 15 years ago, research on graphene-based systems has grown exponentially. Graphene exhibits unique physical properties, for instance, the presence of massless Dirac fermions in a lattice of stronger covalent bonds and frequency-independent optical conductivity, which may help to realize exotic fundamental science and advanced technologies.
So far, graphene has been grown on a multitude of substrates exhibiting interesting properties. In some cases, the graphene layer has minimal link with the substrate. Experiments have revealed enigmatic properties of the Dirac fermions near the band crossing, called Dirac point, at the K point of the Brillouin zone. For example, Angle-Resolved PhotoEmission Spectroscopy (ARPES) data of graphene grown on SiC, shown in Fig. 1a, exhibit large momentum independent intensities near Dirac point as if the top and bottom of the Dirac cone are shifted away from each other. Some studies interpreted these results as a gapped Dirac cone with anomalous in-gap intensities as schematically shown in Fig. 1b. The presence of electron correlation renormalizes the dispersion as shown by red lines. Other proposals involve plasmaron bands where plasmon excitations in addition to photoexcitation of electrons leads to a shifted Dirac cone. The shifted and the pristine Dirac cones appear as a diamond shaped structure around the Dirac point as shown in Fig. 1c.
In order to address this enigmatic scenario, A. Pramanik, S. Thakur and colleagues from India, Italy and Germany performed a detailed polarization dependent ARPES investigation at the BaDElPh beamline at Elettra. Each branch of the Dirac cone was probed selectively using s– and p-polarized synchrotron light. The spectra shown in Fig. 2a,b reveal clearly dispersive bands near the Dirac point.
Read more on the Elettra website
Image: (a) Typical ARPES spectra of graphene on SiC along the ΓKM direction of the Brillouin zone; the origin of the momentum axis is shifted to K point. Schematic of (b) anomalous region and (c) plasmaron scenario around the Dirac point. Red curved lines in (b) show bands in the presence of electron correlation. Red Dirac cone in (c) is due to plasmaron bands.