Weyl fermions discovered in another class of materials

A particular type of elementary particle, the Weyl fermions, was first discovered a few years ago. Their specialty: they travel in a neatly arranged material, virtually never allowing them to collide, saving them a lot of energy. This opens up exciting possibilities for the electronics of the future. Until now, Weyl fermions had only been found in certain non-magnetic materials. However, for the very first time, scientists from the PSI Institute at the Paul Scherrer Institute have proven their existence experimentally in another type of material: a paramagnetic with intrinsic slow magnetic fluctuations. This discovery also shows that it is possible to manipulate Weyl fermions with small magnetic fields, potentially allowing their use in spintronics, a promising development of electronics for a new computer technology. The researchers published their results in the scientific journal Progress of science.

Among the approaches that could pave the way for the energy-efficient electronics of the future, the Weyl fermions could play an intriguing role. Found experimentally only inside materials called quasi-particles, they behave like particles without mass. Predicted theoretically in 1929 by the mathematician Hermann Weyl, their experimental discovery by PSI scientists only occurred in 2015. Until now, the Weyl fermions had only been observed that in some non-magnetic materials. However, a team of PSI scientists and researchers in the United States, China, Germany, and Austria also found them in a specific paramagnetic material. This discovery could bring the potential use of Weyl fermions closer to future computing technologies.

Looking for slow magnetic fluctuations

"The difficult part," said Junzhang Ma, a postdoctoral researcher at PSI and first author of the new study, "was to identify a suitable magnetic material in which to look for these Weyl fermions." For years, although the accepted theoretical hypothesis has been that in some magnetic materials the Weyl fermions can exist on their own, experimental proof is still lacking, in spite of the considerable efforts of several research groups in the field. whole world. The team of PSI scientists then had the idea to focus on a specific group of magnetic materials: paramagnetic slow magnetic fluctuations.

"In specific paramagnetic materials, these intrinsic magnetic fluctuations could be enough to create a pair of Weyl fermions," says Ming Shi, a professor in the same research group as Ma: the Spectroscopy of Novel Materials group. "But we realized that the fluctuations had to be slow enough for Weyl's fermions to appear, so identifying what material could have slow enough magnetic fluctuations had become our main challenge."

Since the characteristic time of magnetic fluctuations is not a characteristic that can be verified in a reference work for each material, it took some time and effort for researchers to find a suitable material for their experiment. A model analysis in theoretical physics also carried out at PSI helped them identify a promising candidate for slow magnetic fluctuations: the material with the chemical notation EuCd₂As₂: Europium-Cadmium-Arsenic. And indeed, in this paramagnetic material, scientists have been able to experimentally prove the Weyl fermions.

Measurements with muons and X-rays

Scientists used two of PSI's large research facilities for their experiments: first, they used the Swiss muon source (SμS) to measure and better characterize the magnetic fluctuations of their materials. They then visualized the Weyl fermions with an X-ray spectroscopy method at the Swiss Light Source SLS.

"What we have proven here is that weyl fermions can exist in a wider range of materials than previously thought," says Junzhang Ma. The scientists' research is therefore expanding considerably. the range of materials considered viable in the search for materials adapted to the electronics of the future. In a developmental area called spintronics, Weyl fermions could be used to carry information with much greater efficiency than electrons of current technology.