ORNL's Andrew Christianson (left) and Stuart Calder (right) conducted neutron diffraction studies at the lab's High Flux Isotope Reactor to clearly define the magnetic order of an osmium-based material. Photo: ORNL/Genevieve Martin.Neutron and X-ray studies by a team of scientists led by the Department of Energy's Oak Ridge National Laboratory (ORNL) and the University of Tennessee have revealed that an elusive massless particle could exist in a magnetic crystal structure.
In a paper published in Nature Communications, the research team studied a material containing the dense element osmium and documented two conditions required for the presence of Weyl fermions – massless particles predicted in 1929 and observed experimentally for the first time in 2015. Researchers are looking for other materials that could host these particles, with the goal of harnessing 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, first author of the paper. "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, a DOE Office of Science User Facility at ORNL, to determine the magnetic order of an osmium-based material with a pyrochlore crystalline structure. They discovered it possesses an ‘all-in, all-out’ magnetic order – one of two properties required for a 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 second property is strong spin-orbit coupling, which 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 even though the osmium in this material is a heavy and dense element, it’s electron configuration was thought to remove spin-orbit effects. Now though, using X-ray analysis at the Advanced Photon Source, a DOE Office of Science User Facility at Argonne National Laboratory, the researchers have uncovered evidence of strong spin-orbit coupling in the osmate pyrochlore material.
"It's expected that the spin-orbit coupling effect in osmium should be suppressed or ignored in this pyrochlore material," Calder explained. "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."
This story is adapted from material from ORNL, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.