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Origin of the flat band in heavily Cs-doped graphene

Since the discovery of graphene more than 15 years ago, the material has been a cornerstone of 2D matter research. In recent years, twisted bilayer graphene has driven interest in flat electronic bands as the resulting high density of states (DOS) at the Fermi level can drive many interesting phase transitions due to many-body interactions. However, as twisted bilayer graphene only acquires these flat bands at a few “magic angles” it can only be produced via flake exfoliation techniques. This inherently limits experimental investigation to microscopic techniques. Finding ways to introduce flat bands at the Fermi edge using scalable techniques for big sample sizes is thus important to investigate the many-body effects introduced by flat bands with high resolution spectroscopy methods.
Using chemical doping via deposition of a large amount of Cs on a monolayer of graphene grown on an Ir(111) surface, N. Ehlen, M. Hell and colleagues from Germany, Italy, Indonesia, Japan, and France were able to introduce a flat band right at the Fermi energy in large samples with size >1 cm2. The flat band formation was characterized using angle-resolved photoemission spectroscopy (ARPES) performed at the BaDElPh beamline at Elettra. Compared with pristine monolayer graphene, the data reported in Fig. 1 show a strong n-type doping of the graphene layer. Additionally, the graphene bands are zone-folded into a 2x2 superstructure. This is in line with the observed Cs derived partially filled electron-bands, which are also observed in this 2x2 superstructure state. A flat electronic band is visible in the region of reciprocal space where Cs and graphene bands interact.
Combining the experimental results with tight-binding (TB) and density functional theory (DFT) calculations, it could be shown that the interaction of the doped graphene bands with the alkali metal bands from the Cs on the surface is responsible for the flat band formation. The Cs layer on top of the graphene forms a 2x2 phase and imprints this superstructure onto the graphene band. Additionally, charge transfer from the Cs to the graphene layer leads to a strong doping of graphene close to the saddle point instability. This makes it possible for the Cs and graphene bands to hybridize, in turn further flattening the graphene band at the Fermi level. Importantly, the interaction with the Cs derived band turns the saddle point of the pristine graphene bands into an extremum. Using a simple two-band model, it could be shown that the DOS diverges with 1/sqrt(E) as expected for an extremum, rather than the slower divergence of ln(E) as expected for a saddle point. The stronger divergence leads to a higher maximum DOS that could drive phase transitions. Indeed, using Thomas-Fermi theory and the Stoner criterion in combination with the experimentally determined DOS, a ferromagnetic instability is predicted for this material.
The experiments highlight the strength of combining ARPES with theoretical calculations for disentangling different effects that play a role in flat band formation and electronic phase transitions.

Figure 1.    (a) ARPES spectra of Cs doped monolayer graphene. (b) High resolution ARPES of the flat band region indicated by the red rectangle in (a). (c) Fermi surface plot tiled from the 2x2 Brillouin zone. The different bands are indicated in different colors. The original 1x1 Brillouin zone is indicated in light blue, the zone-folded 2x2 Brillouin zones in light brown. The red rectangle denotes again the observed flat band.


This research was conducted by the following research team:

Niels Ehlen1, Martin Hell1, Giovanni Marini2, Eddwi Hesky Hasdeo3, Riichiro Saito4, Yannic Falke1, Mark Oliver Goerbig5, Giovanni Di Santo6, Luca Petaccia6, Gianni Profeta2, Alexander Grüneis1
1 II. Physikalisches Institut, Universität zu Köln, Köln, Germany
Department of Physical and Chemical Sciences and SPIN-CNR, University of L’Aquila, Coppito, Italy
Research Center for Physics, Indonesian Institute of Sciences, Kawasan Puspiptek Serpong, Tangerang Selatan, Indonesia
Department of Physics, Tohoku University, Sendai, Japan
Laboratoire de Physique des Solides, CNRS UMR 8502, Université Paris-Saclay, Orsay, France
Elettra Sincrotrone Trieste, Trieste, Italy

Contact persons:

Niels Ehlen, email: ehlen@ph2.uni-koeln.de
Luca Petaccia, email: luca.petaccia@elettra.eu
Alexander Grüneis, email: grueneis@ph2.uni-koeln.de


N. Ehlen, M. Hell, G. Marini, E.H. Hasdeo, R. Saito, Y. Falke, M.O. Goerbig, G. Di Santo, L. Petaccia, G. Profeta, A. Grüneis; Origin of the flat band in heavily Cs-doped graphene, ACS Nano 14, 1055 (2020); DOI: 10.1021/acsnano.9b08622

Last Updated on Tuesday, 12 May 2020 08:07