Graphene on Nickel: Electrons behave like light
In a graphene sheet on nickel, every other
carbon atom is strongly bonded to the nickel atom
which it sits on top of while its neighboring
carbon atoms do not face nickel atoms.
This atomic arrangement breaks the original
lattice symmetry. © STM, A. Varykhalov, HZB
Dr. Andrei Varykhalov and his colleagues in the group of Prof. Dr. Oliver Rader investigated at BESSY II the electronic properties of nickel coated with graphene and achieved an astonishing result. They could show that the conduction electrons of the graphene behave rather as light than as particles. Physicists had originally expected such behavior only for freestanding graphene layers which show a perfect honeycomb structure and not for graphene on nickel which disturbs the perfect hexagonal symmetry. Their results are supported by calculations of two theoretical groups using novel concepts. Their report was published in the open access journal, Phys. Rev. X, the new top journal of the Physical Review.
Employing photoelectron spectroscopy at BESSY II, the physicists were able to establish so-called Dirac cones of massless fermions, which prove the light-like behavior. After their experiments, they could enlist two theoretical groups for supporting their results by contributing new explanations to today's publication. “These results are surprising” says Varykhalov, the reason being that the nickel atoms interact in two different and mutually compensating ways with the carbon atoms of the graphene. On the one hand, they destroy the perfect hexagonal symmetry of the graphene lattice. On the other hand they provide the graphene layer with extra electrons - which compensates for the “damage” inflicted upon the graphene by disturbing the lattice. “We uncovered a fundamental mechanism that is interesting for possible applications” says Varykhalov adding that graphene is usually supported by such a substrate and that the extra electrons for “healing” could as well be supplied by an electrical voltage.
http://prx.aps.org/
A. Varykhalov et al. , Phys. Rev. X 2, 041017
- A new way to control the magnetic properties of rare earth elementsThe special properties of rare earth magnetic materials are due to the electrons in the 4f shell. Until now, the magnetic properties of 4f electrons were considered almost impossible to control. Now, a team from HZB, Freie Universität Berlin and other institutions has shown for the first time that laser pulses can influence 4f electrons- and thus change their magnetic properties. The discovery, which was made through experiments at EuXFEL and FLASH, opens up a new way to data storage with rare earth elements.
- HZB magazine lichtblick - the new issue is out!In his search for the perfect catalyst, HZB researcher Robert Seidel is now getting a tailwind – thanks to a ERC Consolidator Grant. In the cover story, we explain why the X-ray source BESSY II plays an important role for his research.
- BESSY II shows how solid-state batteries degradeSolid-state batteries have several advantages: they can store more energy and are safer than batteries with liquid electrolytes. However, they do not last as long and their capacity decreases with each charge cycle. But it doesn't have to stay that way: Researchers are already on the trail of the causes. In the journal ACS Energy Letters, a team from HZB and Justus-Liebig-Universität, Giessen, presents a new method for precisely monitoring electrochemical reactions during the operation of a solid-state battery using photoelectron spectroscopy at BESSY II. The results help to improve battery materials and design.