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Posted: Mar 11, 2015
Theoretical physicists show topological states in multi-orbital HgTe honeycomb lattices
(Nanowerk News) Graphene is a form of carbon in which the atoms are connected in a honeycomb structure. The possible ‘holy grail’ has this same structure, but is made of nanocrystals of mercury and tellurium. In their paper (Nature Communications, "Topological states in multi-orbital HgTe honeycomb lattices"), the theoretical physicists show that this material combines the properties of graphene with the qualities graphene misses. At room temperature, it is a semiconductor instead of a conductor, so that it can be used as a field-effect transistor. And it fulfils the conditions required to realise quantum spintronics, because it may host the quantum spin Hall effect at room temperature.
Graphene, which was produced for the first time in 2003, is the first material discovered in which electrons move as if they have no mass. This is caused by the honeycomb structure of the Carbon atoms, which induces the electrons to behave as relativistic particles. However, it cannot realise the exotic quantum spin Hall effect, not even at very low temperatures. In their search for the holy grail, the challenge for the theoretical physicists was to find a way to shape a material that could have the potential to realise the quantum spin Hall effect at room temperature in a honeycomb structure.
Honeycomb nanoribbon formed by the HgTe nanocrystals. Hg atoms are in yellow, Te atoms are in grey. The arrows along the ribbon indicate the electron propagation in the helical edge states present in the quantum spin Hall phase. Red and blue colours correspond to top and bottom edge for spin up, bottom and top edge for spin down, respectively.
The quantum spin Hall effect, which was predicted in 1971, was only realised experimentally in 2006 by Prof. Laurens Molenkamp of the University of Würzburg and his team. They used mercury telluride/cadmium telluride quantum wells at a very low temperature. This inspired the theoretical physicists to design several honeycomb structures of mercury telluride nanocrystals and calculate their properties. Several structures turned out to have all the properties of the holy grail they were looking for. At Utrecht University, Prof. Daniël Vanmaekelbergh has already managed to synthesize this kind of honeycomb structures by using cadmium-selenide nanocrystals.
“However, at the moment Prof. Laurens Molenkamp is the only expert in the world working with mercury telluride. So we are happy that he is very interested in synthesizing the honeycomb structures we designed with mercury telluride”, says Prof. Cristiane Morais Smith from Utrecht University. “Although it is not yet possible to realise it experimentally, he expects that the technology necessary will be available within a short time, given the developments that are going on in his lab right now. If we succeed in synthesizing it and the material indeed exhibits the unique combination of exotic properties at room temperature as we predicted, a field of fundamental research and technological innovations opens up that lies beyond our imagination.”
For one thing, it could be used in spintronics, a technology that may be the next step in speeding up computers and the Internet. In spintronics, the electron ‘spin’ is used instead of the electric charge. Spin up and spin down are used to describe whether electrons rotate clockwise or counter-clockwise. If all electrons with spin up move to the left and all electrons with spin down to the right, then they create a spin current instead of an electric current. Spin currents can interact with nanomagnets and lead to applications in the context of fast reading and writing of magnetic memories.