Doped epitaxial graphene close to the Lifshitz transition

Graphene, an spbonded sheet of carbon atoms, is still attracting lots of interest almost 15 years after its discovery. Angle-resolved photoemission spectroscopy (ARPES) is a uniquely powerful method to study the electronic structure of graphene and it has been used extensively to study the coupling of electrons to lattice vibrations (phonons) in doped graphene. This electron-phonon coupling (EPC) manifests as a so-called “kink” feature in the electronic band structure probed by ARPES. What is much less explored is the effect of EPC on the phonon structure. A very accurate probe of the phonons in graphene is Raman spectroscopy.
M.G. Hell and colleagues from Germany, Italy, Indonesia, and Japan combined ARPES (carried out at the BaDelPhbeamline – see Figure 1) with low energy electron diffraction (LEED) and Raman spectroscopy (carried out at the University of Cologne in Germany) in a clever way to fully understand the coupled electron-phonon system in alkali metal doped graphene. LEED revealed ordered (1×1), (2×2), and (sqrt3xsqrt3)R30°adsorbate patterns with increasing alkali metal deposition. The ARPES analysis yielded not only the carrier concentration but also the EPC coupling constant. Ultra-High Vacuum (UHV) Raman spectra carried out using identically prepared samples with the very same carrier concentrations provided the EPC induced changes in the phonon frequencies.

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Image:  Top: ARPES spectra along the Γ-K-M high symmetry direction of the hexagonal Brillouin zone for Cs doped graphene/Ir(111) with increasing Cs deposition. The Dirac energy ED and the observed LEED reconstruction are also indicated. Bottom: Corresponding Fermi surfaces at the indicated charge carrier concentration.