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Possible superconductivity in Ca-doped graphene


Superconductivity, i.e. the transport of electric current without loss, has intrigued material physicists ever since on the pursuit of higher and higher critical temperatures. The superconducting mechanism is still debated and remains a major open question in modern condensed-matter physics. For one class of materials it has been recognized that the phonons (vibration quanta of a material) mediate the coupling between electrons in the superconducting state below the critical temperature.
Whenever a new material is discovered, one of the main questions is whether it is superconducting as well. The exotic properties of graphene, a single-atom thick carbon layer, discovered in 2004 and considered so significant that got a Nobel prize in 2010. However, although the graphene close relatives graphite and fullerenes can be made superconducting by doping with electrons still there are no experimental reports about graphene superconductivity.

Figure 1.  ARPES spectra of maximally Ca-doped graphene. The black dotted line denotes the ARPES intensity maxima. Upper-left: Fermi surface and electrons transferred per C atom (values inside the contour). Lower-left: ARPES scan along the ΓKM high symmetry direction in the vicinity of K point (at 1.7Å-1). The number gives the energy of the Dirac point. Right: High resolution ARPES data in the kink region for KM and ΓK directions, respectively. The yellow lines give the bare-particle band structure.

It is therefore interesting to examine the possibility of achieving superconductivity in monolayer graphene using electron doping. In order to uncover the possible superconducting coupling mechanisms, scientists need to resort to powerful measurement techniques based on one of Einstein’s brilliant discoveries: the photoelectric effect. When light interacts with matter it can transfer its energy to an electron and if the photon energy is sufficiently high, the electron is ejected. Measuring the properties of these photoelectrons holds the key to unveil the mystery of the coupling mechanism.
A special flavor of photoemission spectroscopy is angle-resolved photoemission spectroscopy (ARPES) that monitors not only the energy but also the momentum of the photoelectrons. This enables the scientists to extract valuable information on the electronic properties of the material by determining the energy and the escape angles of the photoelectrons. ARPES is extremely attractive for understanding the superconducting coupling as it is sensitive to many-body interactions that electrons undergo in a solid. These can be the interactions with other electrons, magnetic impurities or, as in the present case, vibrations.

Alexander Fedorov and colleagues from Europe and the US made use of the ARPES method at the BaDElPh beamline of Elettra and tested a series of electron dopants (Cs, Rb, K, Na, Li, Ca) for monolayer graphene. In particular, they measured its electronic band structure (see Fig. 1) and extracted a quantity called the Eliashberg function (see Fig. 2) which gives information on how strong different phonons couple to electrons. From these experiments, they could show that not only the vibrations of graphene mediate the coupling but also the dopant’s vibrations play a crucial role. Calcium doped graphene should have the highest superconducting critical temperature of about 1.5 K. Clearly this is a small value compared to fullerenes, superconducting at 33 K, and cuprates that have the record critical temperatures above 100 K. However, graphene offers one huge advantage over many other materials: since it consists only of carbon atoms and is purely 2D material, it can be easily chemically functionalized. It can be grown as a single layer, bilayer, trilayer etc. in various stacking orders yielding a large variety of parameters that can be modified. Moreover, it is susceptible to both electron and hole doping and can be functionalized by covalent, ionic, and substitutional doping. These properties make graphene a perfect system to play around with the subtle effects of number of layers, dopant type, etc.
Therefore, it is believed that, while graphene will not set new record in critical temperatures, the ease by which its properties can be modified will enhance our understanding of superconductivity in general and carbon materials in particular.

Figure 2.  Dopant dependence of electron–phonon coupling. ARPES-derived Eliashberg functions for all dopants along the KM direction. The two peaks at higher energies are related to C-C vibrations while the extra low-energy peak is due to dopant's vibrations.

 

This research was conducted by the following research team:

Alexander V. Fedorov1,2, Nikolay I. Verbitskiy3,4, Danny Haberer1,5, Claudia Struzzi6, Luca Petaccia6, Dmitry Usachov2, Oleg Y. Vilkov2, Denis V. Vyalikh2,7, Jörg Fink1, Martin Knupfer1, Bernd Büchner1, Alexander Grüneis1,3

1     IFW Dresden, Dresden, Germany
2     St Petersburg State University, St Petersburg, Russia
3     University of Vienna, Vienna, Austria
    Moscow State University. Moscow, Russia
5     University of California, Berkeley, USA
6     Elettra Sincrotrone Trieste, Trieste, Italy
7     TU Dresden, Dresden, Germany

Contact person:
Alexander V. Fedorov : a.fedorov@ifw-dresden.de
Luca Petaccia : luca.petaccia@elettra.eu
Alexander Grüneis : grueneis@ph2.uni-koeln.de

 

Reference

A.V. Fedorov, N.I. Verbitskiy, D. Haberer, C. Struzzi, L. Petaccia, D. Usachov, O.Y. Vilkov, D.V. Vyalikh, J. Fink, M. Knupfer, B. Büchner, and A. Grüneis, “Observation of a universal donor-dependent vibrational mode in graphene”, Nat. Commun. 5, 3257 (2014);  DOI: 10.1038/ncomms4257
Last Updated on Thursday, 15 May 2014 10:06