Are Weyl fermions really massless particles

Massless quasiparticle behaves like "slow light"

Vienna - Two years ago researchers from Rice University in Texas (USA) and the Technical University (TU) Vienna succeeded for the first time in detecting quasiparticles in special materials. These particles move very slowly in these crystals, although they have no mass. This opens the door to new technological applications and ultimately also to completely new physics, as the researchers now report in the specialist journal "Pnas".

Almost 90 years ago, the German mathematician and physicist Hermann Weyl (1885-1955) theoretically predicted the existence of exotic, massless particles, so-called Weyl fermions. In 2015 they were finally proven experimentally for the first time. However, not as free particles that can move freely through space, but as quasiparticles that only exist inside a solid.

Excited systems

Quasiparticles are quantum mechanical objects that consist of a large number of interacting particles, but whose collective state is described as if they would together form a new particle. Similar to the propagation of a water wave, it is not the individual particles that move. Rather, it is their common stimulus that spreads in the form of a wave.

While free, massless particles must always move at the speed of light according to the laws of relativity, massless quasiparticles can also be slower. When they investigated the properties of a special crystal made from the elements cerium, bismuth and palladium, the researchers have now discovered particularly slow Weyl fermions in their current study.

1,000 meters per second

"In the crystal structure we examined, the electrons are highly correlated and interact very strongly with one another. As a result, the Weyl fermions move extremely slowly," explained co-author Silke Bühler-Paschen from the Institute for Solid State Physics at the Vienna University of Technology. "This way the effect can be controlled much better." At 1,000 meters per second, their speed is only around three thousandths of a per thousand of the speed of light in a vacuum.

According to the researcher, this also makes the new system interesting for technological applications. Since Weyl fermions are hardly scattered in the material, they can conduct electricity with almost no loss. In addition, their spin is particularly robust, which could lead to new applications that use the spin of electrons for data processing in addition to the charge.

Fault tolerant quantum computing

The new discovery could also have an impact on the development of quantum computers. "According to theoretical predictions, quantum information in such materials would be processed globally instead of locally," says Bühler-Paschen. "That could enable fault-tolerant quantum computing in the future." To do this, however, the effect must first be transferred to a superconducting material. (APA, red, December 19, 2017)