Enigmatic Dirac fermions in graphene

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.

Figure 1 by Pramanik et al.

Figure 1: (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.

Circularly polarized light (σ-pol) probes both the branches together: the overlap of the Dirac bands in close proximity near Dirac point (separation smaller than the linewidth) leads to the anomalous intensities. The data match well with the DFT results shown by solid lines in Fig. 2c. Evidently there is no gap and/or plasmaron bands near Dirac point and the internal symmetries are protected in graphene on SiC.

Figure 2 by Pramanik et al.

Figure 2. ARPES data of graphene on SiC along ΓKM using (a) σ-polarized, (b) p-polarized, and (c) circularly polarized light. (d) Similar ARPES data of 5% graphitic B-doped sample using circularly polarized light. The origin of the momentum axis is shifted to K point. Dashed lines show the peak positions and solid lines are the DFT results. The Dirac point εD is also indicated. (Adapted from Phys. Rev. Lett. 128, 166401 (2022) © 2022 American Physical Society.)

To verify the robustness of such symmetry protection, 5% graphitic B doped graphene/SiC was also probed by ARPES and data are shown in Fig. 2d. Curiously, even this case exhibits dispersive bands across the Dirac point. The Dirac fermionic behavior remains protected although the mobility and density of charge carriers changed influencing the electronic properties significantly. Internal symmetries are protected in the doping regime studied. Clearly, graphene on SiC provides an excellent platform to realize new paradigm involving exotic science and advanced technology.

This research was conducted by the following research team:

Arindam Pramanik1,*, Sangeeta Thakur1,2,*, Bahadur Singh1, Philip Willke3, Martin Wenderoth3, Hans Hofsäss4, Giovanni Di Santo2, Luca Petaccia2 and Kalobaran Maiti1

1Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Mumbai, India
2Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy
3IV. Physikalisches Institut, Georg-August-Universität Göttingen, Göttingen, Germany
4II. Physikalisches Institut, Georg-August-Universität Göttingen, Göttingen, Germany
* These authors contributed equally

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A. Pramanik, S. Thakur, B. Singh, P. Willke, M. Wenderoth, H. Hofsäss, G. Di Santo, L. Petaccia and K. Maiti, “Anomalies at the Dirac point in graphene and its hole-doped compositions”, Phys. Rev. Lett. 128, 166401 (2022), DOI: 10.1103/PhysRevLett.128.166401

Last Updated on Thursday, 08 September 2022 10:51