Elusive particle shows up in ‘semimetal’

A kind of particle first predicted to exist before the discovery of Pluto has been spotted on Earth within a compound of tantalum and arsenic.

The newly discovered particles, known as Weyl fermions, resemble massless electrons that dart around and through the material in unusual and exciting ways, researchers report July 16 in Science.

“It’s definitely a big deal,” says Leon Balents, a condensed matter theorist at the University of California, Santa Barbara.

The particles’ behavior makes tantalum arsenide a metal-like compound that shares desirable features with graphene and topological insulators, materials that have attracted a torrent of research attention over the last decade or so. “There are a lot of reasons to be interested in these materials,” Balents says.

Materials like tantalum arsenide could enable future electronic devices to feature fast-moving current that easily circumvents bumps and valleys in its path. Physicists will be able to study the properties of the material-bound particles to explore the possibility that free-floating varieties of Weyl fermions exist.

Electrons, neutrinos and a host of other subatomic particles belong to a family called fermions. All the known fermions behave according to equations devised in 1928 by English theoretical physicist Paul Dirac. But at least in theory, there are two other kinds of fermions, both proposed soon afterward: Majorana fermions and Weyl fermions. Unlike Dirac and Majorana fermions, members of the Weyl class —named after German mathematician and physicist Hermann Weyl —are massless.

Physicists have failed to discover Weyl fermions in any particle detector or accelerator. But researchers have suggested that collective interactions of electrons in certain materials would make propagating ripples of energy that behave just as Weyl fermions would in free space.

Earlier this year, condensed matter physicists Su-Yang Xu and Zahid Hasan at Princeton University and colleagues proposed that tantalum arsenide could host Weyl fermions. The researchers fired radiation at crystals of the compound to probe the energies and motion of the electrons inside. High-energy X-rays that pierced deep into the material revealed the signature of massless particles that fit the profile of Weyl fermions. “I’m kind of amazed that someone was able to really see these things experimentally so quickly,” Balents says.

By housing Weyl fermions, tantalum arsenide becomes the first experimentally confirmed Weyl semimetal, a metal-like material with exotic and potentially useful features. Weyl semimetals resemble topological insulators, materials that are insulating inside but allow electrons to run laps undisturbed around the surface (SN: 5/22/10, p. 22). Despite lacking an insulating interior, tantalum arsenide also has protected high-speed electron highways on its surface. The twist, Xu says, is that the surface electrons don’t race around a closed track; instead, they move from one side to the other before disappearing into the bulk and reemerging on the opposite surface.

Weyl semimetals also resemble atom-thick sheets of carbon called graphene, Balents says. Both materials allow electrons to zip around at tremendous speed and behave as if they were massless (SN: 8/13/11, p. 26). All these features make Weyl semimetals an enticing candidate for future electronics, Hasan says, and for shuttling electrical current without resistance at room temperature.

The discovery of the second of the three types of fermions may lead to finding the third, Xu says. In recent years physicists have found hints but no definitive evidence of Majorana fermions, which are theorized to be their own antimatter counterparts (SN: 11/15/14, p. 8). The properties of Weyl semimetals make them good candidates to produce not only Weyl fermions but Majorana particles as well, Xu says.