Spins in Graphene with a Hedgehog Texture
The hedgehog-configuration of the spins and the Fermi-Level is shown. Illustration Thomas Splettstößer/HZB
HZB researchers demonstrate a fundamental property of the electron spin in graphene
HZB researchers have been experimenting for quite some time with graphene, a material famous for its highly mobile electrons. They intend to impose an additional property on the graphene. This property is a coupling between the direction of motion of these electrons and their angular momentum, in other words: their spin. This is, however, an exclusive property of heavy elements, for example gold. Graphene consists of carbon and is too light to this end. HZB researchers, however, are experts in depositing gold atoms underneath a graphene layer in a controlled way. In this way, one can indeed create peculiar spin textures which have become known as the "Rashba effect". Nevertheless, only spin textures within the graphene plane had been possible. Now Dr. Andrei Varykhalov and co-workers succeeded to turn the spin also out of the plane.
They achieve this by turning it successively out of the plane towards the surface normal, an arrangement as with the spikes of a hedgehog. The researchers verified this with spin-resolved photoelectron spectroscopy at BESSY II.
Indeed, such hedgehog structures are known, for example in nuclear physics. These are singular points which, in principle, would contradict the prohibition of magnetic monopoles, according to Gauss. Here, Varykhalov remarks that in graphene, everything is doubled because its honeycomb-type crystal structure consists of two equivalent atomic lattices. Indeed, also the hedgehog has a kind of anti-hedgehog, and both together comply with the monopole prohibition.
That both hedgehogs cancel each other does not mean that they do not have physical consequences, on the contrary, explains Prof. Oliver Rader, the head of the department. In fact, the physicists suggested in their study a spintronic device which uses the hedgehog structure to realize a very efficient spin filter. In the spin filter, the spins are deflected to the left and right, respectively. The resulting spin current is in principle lossless and could in the future reduce the energy consumption in the information technology.
The effect in the graphene has a couple of years ago been predicted by a group from Budapest. Andros Kormányos explains that the hedgehog and the anti-hedgehog had already been present in the previously realized graphene systems. However, they were inseparably superimposed. Only by breaking of the sublattice symmetry, which Varykhalov achieved by chosing a substrate crystal of a lower symmetry, the hedgehog could be separated from the anti-hedgehog.
The study is published by the renowned journal Nature Communications (27. July 2015). The underlying prediction appeared in 2011 in Phyisical Review B.
Publication: A. Varykhalov, J. Sánchez-Barriga, D. Marchenko, P. Hlawenka, P.S. Mandal & O. Rader,
Tunable Fermi level and hedgehog spin texture in gapped graphene
NATURE COMMUNICATIONS | 6:7610 | DOI: 10.1038/ncomms8610
Rakyta, P., Kormányos, A. & Cserti, J. Effect of sublattice asymmetry and
spin-orbit interaction on out-of-plane spin polarization of photoelectrons.
Phys. Rev. B 83, 155439 (2011)
- Peat as a sustainable precursor for fuel cell catalyst materialsIron-nitrogen-carbon catalysts have the potential to replace the more expensive platinum catalysts currently used in fuel cells. This is shown by a study conducted by researchers from the Helmholtz-Zentrum Berlin (HZB), Physikalisch-Technische Bundesanstalt (PTB) and universities in Tartu and Tallinn, Estonia. At BESSY II, the team observed the formation of complex microstructures within various samples. They then analysed which structural parameters were particularly important for fostering the preferred electrochemical reactions. The raw material for such catalysts is well decomposed peat.
- Helmholtz Investigator Group on magnonsDr Hebatalla Elnaggar is setting up a new Helmholtz Investigator Group at HZB. At BESSY II, the materials scientist will investigate so-called magnons in magnetic perovskite thin films. The aim is to lay the foundations for future terahertz magnon technology: magnonic devices operating in the terahertz range could process data using a fraction of the energy required by the most advanced semiconductor devices, and at speeds up to a thousand times faster.
- The future of corals – what X-rays can tell usThis summer, it was all over the media. Driven by the climate crisis, the oceans have now also passed a critical point, the absorption of CO2 is making the oceans increasingly acidic. The shells of certain sea snails are already showing the first signs of damage. But also the skeleton structures of coral reefs are deteriorating in more acidic conditions. This is especially concerning given that corals are already suffering from marine heatwaves and pollution, which are leading to bleaching and finally to the death of entire reefs worldwide. But how exactly does ocean acidification affect reef structures?
Prof. Dr. Tali Mass, a marine biologist from the University of Haifa, Israel, is an expert on stony corals. Together with Prof. Dr. Paul Zaslansky, X-ray imaging expert from Charité Berlin, she investigated at BESSY II the skeleton formation in baby corals, raised under different pH conditions. Antonia Rötger spoke online with the two experts about the results of their recent study and the future of coral reefs.