Magnetic molecules manipulate surface of topological insulator

11 January 2018

Researchers at the University of Oxford have demonstrated, for the first time, how to use molecules to manipulate the surface of a ‘topological insulator’, an exotic material with unusual electronic properties. Understanding and manipulating these properties is a notable step towards the innovative information transfer and storage promised by ‘spintronics’.

Topological insulators are distinguished by the fact that, although the bulk of the material is insulating, the surfaces are conductive, allowing electrons to flow along them. However, these electrons are protected by a symmetry that ‘couples’ or locks together their momentum and spin, so that if one is to change the other must change at the same time. As a result, the electrons can only scatter off something that can change their spin as well as their momentum – that is, something magnetic. This result from University of Oxford physicists demonstrates a new class of experiments designed to confirm of this effect.

Moon-Sun Nam and her team from the Quantum Materials group demonstrated the effect of magnetic materials on the surface of a topological insulator by ‘decorating’ the surface of samarium hexaboride crystals, a candidate topological insulator, with either magnetic or non-magnetic molecules. The two molecules chosen, Ga7Zn and Cr7Zn, with structures as shown in the figure below, are very similar electrostatically, but Cr7Zn will also interact magnetically with the surface of the topological insulator.

A photograph of one of the samarium hexaboride crystals, with contacts and 25 µm gold wires in place for measuring its electrical properties. The contacts are outlined in blue for clarity.

Following the addition of the surface decoration, the resistance of the crystal with extra Ga7Zn molecules was found to be lower than that of the crystal with Cr7Zn molecules. This result matches the theoretical prediction that the magnetic molecules enable the scattering of electrons at the surface of the topological insulator in a way that the non-magnetic molecules do not. Furthermore, it agrees with the underlying principle that the spin and momentum of these surface electrons are coupled.

The experiments and results of this work demonstrate an effective method for modifying the surface properties of a topological insulator. This is an important step towards being able to control and exploit topological insulators in ‘spintronic’ devices, which utilise not only charge but also the spin of electrons to store information. Spintronics offers significant improvements in data storage and transfer efficiencies, and topological insulators could ultimately play a leading role.

M.-S. Nam et al. (2018). How to probe the spin contribution to momentum relaxation in topological insulators. Nature Communications 9, 56. https://doi.org/10.1038/s41467-017-02420-4