Magnetic Material could host wily Weyl fermions

An elusive massless particle could exist in a magnetic crystal structure, revealed by neutron and X-ray research. The team studied a material containing the dense element osmium and documented 2 conditions required for the presence of Weyl fermions -predicted in 1929 and observed experimentally in 2015. Researchers are looking for other materials that could host them to harness their unique properties in spintronics and advanced computing applications such as quantum computers.

“Once you have a material that hosts these particles, they can behave like electrons but travel much faster since they’re massless,” said ORNL’s Stuart Calder. “Since all of electronics is based on the electron, if you replace electrons with these Weyl fermions, in principle you could have much faster devices.”

The scientists conducted neutron diffraction studies at the High Flux Isotope Reactor, ORNL, to clearly define the magnetic order of an osmium-based material with a pyrochlore crystalline structure. They discovered it has an “all-in, all-out” magnetic order -1 of 2 requirements for this type of material to contain Weyl fermions.”It describes the spins of electrons and how they arrange; they all either point to the center or they all point out,” Calder said. “Neutrons are the standard and the best way to determine magnetic structure. The magnetic peaks in these materials are weak because they have smaller sized spins, so you have to use an instrument like we have here to see them.”

The 2nd criterion is strong spin-orbit coupling, a property of all atoms that describes how an electron’s spin and its motion around an atom are linked. Generally, larger atoms with more electrons exhibit a stronger spin-orbit effect. But the osmium in this material, even though it is a heavy and dense element, has an electron configuration thought to remove spin-orbit effects. They uncovered evidence of strong spin-orbit coupling in the osmate pyrochlore material using X-ray analysis at the Advanced Photon Source.

“It’s expected that the spin-orbit coupling effect in osmium should be suppressed or ignored in this pyrochlore material,” Calder said. “But this was the first time anyone measured an osmium-based material with this X-ray technique. The point of the X-rays was to look for signatures of strong spin-orbit coupling and that’s what we saw.”

Calder cautions that the team’s research is not direct evidence of Weyl fermions in an osmate material, but it does suggest the material is a potential host.”It shows the magnetic ground state of the material and presence of strong spin-orbit coupling that are required to have these Weyl fermions,” he said. “A lot of people are only looking at iridium-based materials for hosting the spin-orbit coupling effect that can give you new physics. This shows osmium-based materials are important too.” https://www.ornl.gov/news/ornl-research-finds-magnetic-material-could-host-wily-weyl-fermions