At a surface or interface the electron spin can form specific patterns but it remains in the surface plane. Hemholtz Zentrum Berlin researchers have now succeeded in turning the spin out of the plane, and they explain why this is a principle property.

If an electron bounces back from an obstruction it runs, as one should think, exactly back the way it came from. Quantum mechanics, however, has its own rules when it comes to electrons and particularly when it comes to electrons in graphene. When an electron in graphene runs head on against an obstruction and is scattered back, it changes its course by 180 degrees. Its spin, however, should also turn by 180 degrees but it rotates only to be 90 degrees. An electron has to be rotated by 720 degrees to get it back into its original state.

To do this experiment, several preconditions have to be met. First of all, the electron spin property has to be imparted on the graphene. Dr. Andrei Varykhalov and his coworkers have much experience since they succeeded in this in a remarkable experiment in 2008. They squeezed gold atoms underneath the graphene und thereby enhanced the spin-orbit interaction in the graphene by a factor of 10,000. The second precondition is to allow for the 180-degree backscattering. This is challenging since graphene is first and foremost famous for the absence of backscattering.

To this end, Varykhalov and the team created a band gap in the graphene. This means nothing else than sending electrons back by 180 degrees. If both are fulfilled, the spins in this band gap have to be oriented perpendicular to the graphene plane (further away, however) in the plane. The continuous transition between the two has the appearance of the prickles of a hedgehog. Model calculations have been performed by theoreticians from Budapest which confirm the experimental results.

For symmetry reasons the hedgehog structure has to be reversed elsewhere in the graphene. This does not mean that the hedgehog had no influence on the graphene. On the contrary, the so-called valley Hall effect can be used to realize a spin filter. This effect means that the electrons in the graphene are deflected to the right or left depending on which valley they are in. According to the results by Varykhalov and the other researchers, the two valleys correspond to two spin orientations, and the two spins assemble at opposite sides of the graphene sample.

These results were published recently in Nature Communications. They build on previous work published in 2011 in Physical Review B.

Release Date: July 27, 2015
Source: Hemholtz Zentrum Berlin