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Environmental control in graphene: influence of the substrate’s metallicity

Superconductivity in graphite intercalation compounds (GICs) shows a maximum superconducting transition temperature (TC) of 11.5 K for Ca-doped graphite. In the three-dimensional GICs, the dopant pattern is dictated by the delicate equilibrium between Coulomb repulsion of the dopant ions and gain in total binding energy upon formation of ionic bonds. Therefore GICs do not provide sufficient options for engineering the dopant order.
Epitaxial graphene however, provides many opportunities for experimenting with different dopant structures and their impact on properties. Little is known on the influence of the environment on electron-phonon coupling (EPC), in particular of the substrateʼs metallicity. Theoretically, it is expected that a metallic substrate can reduce EPC by screening from substrate electrons. Yet very few dedicated experiments have been performed along this line owing to the difficulty in preparing different interfaces with identical stoichiometries.
An international research team led by Nikolay Verbitskiy and colleagues carried out low energy electron diffraction (LEED) and angle resolved photoemission spectroscopy (ARPES) measurements at the BaDElPh beamline. They discovered that Ba-doped graphene represents a novel and well-ordered BaC8 interface on metallic Au and semiconducting Ge substrates as shown in Fig. 1.

Figure 1. LEED patterns of Ba doped graphene/Ge (a) and graphene/Au (b) substrates indicating the p(2×2) reconstructions. For graphene/Au an additional c(4×2) LEED pattern with weaker spots appears. Corresponding top-view structures (c) on Ge (in red) and (d) on Au (in yellow) substrates.

These phases are distinctly different to the non-superconducting BaC6 GIC. This unique situation for Ba leads to a gapless spectral function of Ba doped graphene and provides a new system to study the influence of the substrate on the interface formation and the electronic properties (see Fig. 2). It was shown that Ba-doped graphene has an attainable value of TC only on a Ge substrate.

Figure 2. High resolution ARPES data in the kink region for (a) Au and for (b) Ge substrates. The black(red) line denotes the measured (bare) energy band structure. The difference between black and red lines provide the real part of the self-energy ReS (red) and blue circles in panel (c) for Au and Ge, respectively) from which the Eliashberg function, which gives information on how strong different phonons couple to electrons, can be extracted. The two peaks at higher energies in the Eliashberg function (smooth curves in panel c) are related to C-C vibrations while the low-energy peak is due to Ba vibrations.

The experimental findings are supported by theoretical modelling, showing significant phonon softening on the semiconducting Ge substrate. This allowed to quantitatively assess the environmental effects for both Au and Ge substrates on superconductivity in graphene. For semiconducting Ge substrates, the doping level and EPC are higher. This study highlights that both dopant order and the metallicity of the substrate can be used to control EPC and hence superconductivity.


 

This research was conducted by the following research team:

Nikolay I. Verbitskiy1,2,3, Alexander V. Fedorov2,4,5, Cesare Tresca6,7, Gianni Profeta6,7, Luca Petaccia8, Boris V. Senkovskiy2,5, Dmotry Yu. Usachov5, Denis V. Vyalikh5,9, Lada V. Yashina10, Andrei A. Eliseev3, Thomas Pichler1 and Alexander Grüneis2 
 
Faculty of Physics, University of Vienna, Austria
2 II. Physikalisches Institut, Universität zu Köln, Germany
3 Department of Materials Science, Moscow State University, Russia
4 IFW Dresden, Germany
5 St. Petersburg State University, Russia
6 Department of Physical and Chemical Sciences, University of L’Aquila, Italy
7 CNR-SPIN, Italy
8 Elettra Sincrotrone Trieste, Italy
9 Institute of Solid State Physics, Germany
10 Department of Chemistry, Moscow State University, Russia




Contact person:

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

 

Reference

N.I. Verbitskiy, A.V. Fedorov, C. Tresca, G. Profeta, L. Petaccia, B.V. Senkovskiy, D.Yu. Usachov, D.V. Vyalikh, L.V. Yashina, A.A. Eliseev, T. Pichler, and A. Grüneis, Environmental control of electron–phonon coupling in barium doped graphene, 2D Materials 3, 045003 (2016); DOI:10.1088/2053-1583/3/4/045003

 
Last Updated on Monday, 12 December 2016 11:34