Spins in Graphene with a Hedgehog Texture

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)