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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 BaDelPh beamline – 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 (1x1), (2x2), 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. 
 

Figure 1.  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. 


In these experiments they could achieve a record charge carrier concentration of 4.4x1014cm-2. At this carrier concentration the Fermi surface is at the onset of a so-called Lifshitz transition. At this transition, the phonons of graphene are strongly renormalized due to the combined effects of EPC and lattice expansion. The authors also observed a strong Fano line shape of the Raman active zone center optical phonon of graphene. Calculations carried out by the authors are able to reproduce the experiments with excellent accuracy and highlight the important contribution of electronic Raman scattering. 
The experiments also show that combining ARPES and Raman in UHV is a new and powerful approach to study the coupled electron-phonon system in 2D materials.

 

 

 

This research was conducted by the following research team:

Martin G. Hell1, Niels Ehlen1, Boris V. Senkovskiy1, Eddwi H. Hasdeo2,3, Alexander Fedorov1, Daniela Dombrowski1,4, Carsten Busse4,5, Thomas Michely1, Giovanni Di Santo6, Luca Petaccia6, Riichiro Saito2, and Alexander Grüneis1

 

1 II. Physikalisches Institut, Universität zu Köln, Köln, Germany
Department of Physics, Tohoku University, Sendai, Japan
Research Center for Physics, Indonesian Institute of Sciences, Tangerang Selatan, Indonesia
Institut für Materialphysik, Westfälische Wilhelms-Universität Münster, Münster, Germany
Fakultät IV Physik, Universität Siegen, Siegen, Germany
Elettra-Sincrotrone Trieste, Trieste, Italy


Contact persons:

Martin G. Hell, email: hell@ph2.uni-koeln.de
Alexander Grüneis, email: grueneis@ph2.uni-koeln.de

Reference

M.G. Hell, N. Ehlen, B.V. Senkovskiy, E.H. Hasdeo, A. Fedorov, D. Dombrowski, C. Busse, T. Michely, G. Di Santo, L. Petaccia, R. Saito, and A. Grüneis; Resonance Raman Spectrum of Doped Epitaxial Graphene at the Lifshitz Transition, Nano Letters 18, 6045 (2018); DOI: 10.1021/acs.nanolett.8b02979. 

 
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Last Updated on Wednesday, 14 November 2018 17:33