New quantum state of matter

Electrons behave in sodium bismuthate as in graphene but in three dimensions

Analyzes at the Advanced Light Source in Berkeley identified the novel quantum state. © LBNL / Roy Kaltschmidt
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Researchers have demonstrated a new quantum state of matter: they first identified a material, sodium bismuthate (NaBiO3), in which electrons behave as in two-dimensional graphs, but in three dimensions. As the scientists in the journal "Science" report, such so-called three-dimensional topological Dirac semi-metals (3DTDS) could in future generate much faster transistors and more compact hard disks.

They are considered the two most exciting and promising new high-tech materials: the graphene, consisting of a two-dimensional network of carbon atoms, in which electrons move as fast as massless particles, as well as so-called topological insulators. These are crystalline materials that are conductive on their surface but behave like non-conductors in their interior. In these crystals, the electrons behave similarly unusually as in the graphene, they correspond to so-called 2D Dirac fermions.

Wanted counterpart in 3D

"The rapid development of graphene and topological insulators has raised the question of whether there are also three-dimensional counterparts with this unusual electron behavior, " explains study leader Yulin Chen of the Lawrence Berkeley Laboratory. Such so-called three-dimensional topological Dirac semi-metals (3DTDS) have been theoretically predicted for some time, but their detection has not been successful.

"A 3DTDS is a natural, three-dimensional counterpart to the graphene, with similar or even better electron mobility and velocity, " Chen said. Materials in such a state produce a linear magnetoresistance that can be orders of magnitude higher than the materials currently used in hard drives. They could also open the door to much more efficient optical sensors, as the researcher explains.

The state of the three-dimensional topological Dirac half-metal arises at the transition from a normal non-conductor to a topological insulator. © LBNL

X-rays reveal electronic structure

Chen and his colleagues have now for the first time actually discovered a material that occupies this theoretically postulated quantum state. They have demonstrated this by analyzing sodium bismuthate, a compound that was considered a potential candidate for a 3DTDS substance, by means of so-called angle-resolved photon electron spectroscopy (ARPES). Behind this Wortunget m hides the irradiation of the material with focused X-rays. These rain electrons on the surface of the samples and encourage them to emit light. The angles and kinetic energies of these photons are measured and give hints to the electronic structure and its behavior inside the material. display

"In the analyzed sodium bismuthate, the conduction and valence bands of the electrons only touch at discrete points, " explains Chen. As a result, electrons can penetrate the insulating gap at these points in a manner that is as fast and easy as in the graphene. They also correspond to Dirac fermions, but not in 2D, as in graphene, but in 3D. As the researchers explain, this state found in sodium bismuthate thus corresponds to a three-dimensional topological Dirac semimetal this unique state of matter is thus experimentally proven for the first time.

Application in future Spintronic technologies

"Such a 3DTDS system could significantly improve efficiency in many applications because of its 3D volume, " explains Chen. In addition, such materials would be even easier to make than graphene: "Growing just one atom of thin graphene layers is still a challenge, it would be easier to create graphene-like 3DTDS objects. However, sodium bismuthate is too unstable to be used in everyday life. However, the researchers hope to find other materials that possess this special quantum state.

In any case, the discovery of sodium bismuthate as a 3DTDS system opens up the opportunity to study the new physical effects and properties associated with this unique condition, the researchers said. That could also promote future electronics technology. Future 3DTDS systems would then become an ideal platform for applications including the spintronic of an electronic system that uses not electrons, but spin, the self-rotation of the electrons. (Science, 2014; doi: 10.1126 / science.1245085)

(Lawrence Berkeley Laboratory, 17.01.2014 - NPO)