Massive and massless charge carriers in an epitaxially strained alkali metal quantum well on graphene
In recent years, heterostructures of different van-der-Waals materials such as Graphene, MoS2 or h-BN have generated a lot of interest due to the many ways the different layers can interact. However, while research on heteroepitaxy of these van-der-Waals layers has been a key point of investigation, growth methods and investigations of hybrid structures composed of van-der-Waals materials and metals are still unexplored. Exploring this venue could have a large impact on electronic structure engineering and growth techniques involving 2D matter as substrates.
Combining angle-resolved photoemission spectroscopy (ARPES) carried out at the BaDElPh beamline in Elettra with Raman spectroscopy and density functional theory (DFT), M. Hell, N. Ehlen and colleagues from Germany, Italy, Austria, the Netherlands and France showed that Cs intercalated bilayer graphene can act as a substrate for the growth of strained Cs films. AB-stacked bilayer graphene was exposed to a large amount of Cs in a ultra-high vacuum (UHV) chamber. After this evaporation ARPES spectra, shown in Fig. 1, reveal four new parabolic electronic bands arising from the Cs grown on the bilayer graphene substrate. The effective mass of the bands is close to the free electron mass and Fermi surface maps show a nearly isotropic behavior in k-space. The four partially occupied electronic bands suggest the growth of a crystalline Cs quantum well with at least four layers. These experimental results were combined with theoretical calculations for the stability of different Cs configurations and DFT calculations of the corresponding band structures. It could be shown that Cs growing along the (111) direction in a fcc-lattice (strained by 11% versus the unstrained structure) with one Cs layer below the bilayer graphene, one Cs layer in between the two graphene layers, and three or four Cs layers growing on top of the graphene have the lowest surface energies and also reproduce the experimentally observed band structure very well.
Investigation of the broadening of the Cs quantum well band with highest binding energy showed a small, constant broadening for low binding energies. The constant broadening implies a small electron-electron scattering in the Cs quantum well, suggesting that the Cs quantum well structure is an experimental realization of a 2D Fermi gas. This agrees with the observed isotropic dispersion of the band with a nearly free band mass.
In the future, the Cs layers could be used as intermittent layers to then wet the surface with conventional metals or semiconductors, opening up new channels for material engineering.
Figure 1. (a) ARPES spectrum along ΓKM direction of pristine bilayer graphene. The four Cs quantum well states are numbered. (b) Focus on the four Cs quantum wells. (c) Momentum distribution cut (MDC) along the red line given in (a) shows a sharp peak with a full width half maximum of 0.009 Å-1. (d) For binding energies below 0.1 eV the imaginary part of the self-energy is constant. (e) Fermi surface of the system, the Cs quantum wells 1, 2 and 4 are indicated by black arrows.
This research was conducted by the following research team:
Martin Hell1,*, Niels Ehlen1,*, Giovanni Marini2, Yannic Falke1, Boris V. Senkovskiy1, Charlotte Herbig1, Christian Teichert1,3, Wouter Jolie1,4, Thomas Michely1, Jose Avila5, Giovanni Di Santo6, Diego M. de la Torre1, Luca Petaccia6, Gianni Profeta2, Alexander Grüneis1
1 II. Physikalisches Institut, Universität zu Köln, Köln, Germany
2 Department of Physical and Chemical Sciences and SPIN-CNR, University of L’Aquila, Coppito, Italy
3 Institute of Physics, Montanuniversität Leoben, Leoben, Austria
4 Institute for Molecules and Materials, Radboud University, AJ Nijmegen, Netherlands
5 Synchrotron SOLEIL & Université Paris-Saclay, Gif sur Yvette, France
6 Elettra Sincrotrone Trieste, Trieste, Italy
* These authors contributed equally
Contact persons:
Niels Ehlen, email:
Luca Petaccia, email:
Alexander Grüneis, email:
