Imitation dark matter axions have arrived. They could reveal the real thing

If imitation is a form of flattery, then scientists are enamored with the axion.

The hypothetical subatomic particle has long eluded scientists. But it’s now been conjured up in imitation form within a thin sheet of material, researchers report April 16 in Nature.

If axions exist, they could explain dark matter, an invisible form of matter inferred from observations of the cosmos. But efforts to spot the particles have been unsuccessful. The newfound axion imitators are the next best thing.

“This is quite impressive work,” says experimental condensed matter physicist Seongshik Oh of Rutgers University in Piscataway, N.J., who was not involved with the research. “This is the first observation of something like axion particles.”

And the quasiparticles’ impersonation is spot-on. “They behave almost exactly the same way as the axion particle,” says chemist Suyang Xu of Harvard University. “So although we haven’t found the axion particle yet, this would allow us a way to basically study the behavior of the elusive, important particle inside a material.”  

The axion imitators arise from the collective behavior of numerous particles within the material, forming what’s known as a quasiparticle. The idea to make these axion quasiparticles, proposed in 2010, went unfulfilled for 15 years. To create them, Xu and colleagues needed just the right substance — thin sheets of a material called manganese bismuth telluride, first created only in 2019.

In this material, electric and magnetic fields are connected: Applying an electric field to a sheet of manganese bismuth telluride induces magnetization. An axion quasiparticle results when the coupling between the electric field and magnetization changes over time, oscillating in a particular way. True axion particles likewise forge a connection between electricity and magnetism: An axion entering a strong magnetic field can be converted into an oscillating electric field, which forms a particle of light, or photon.

To produce the quasiparticle, the researchers used a laser to create a magnon, a magnetic wave that travels through the material. Then they used another laser to probe the magnetization of the material, revealing an oscillation in the coupling between the electric field and magnetization over time — the hallmark of an axion.

Scientists have previously found hints of axion quasiparticles in other materials, but that evidence was indirect. In contrast, the oscillation the researchers observed in the new study is a tell-tale sign of axions. “This, essentially, is the definition of this axion quasiparticle,” says chemist Jianxiang Qiu of Harvard.

What’s more, manganese bismuth telluride could be used to create a detector capable of spotting true axion particles in the wild — if they exist. If an axion enters a magnetic field around the material, it would convert into a photon, which would interact with an axion quasiparticle. This interaction would amplify the photon signal, which would otherwise be too weak to detect, allowing it to be observed.

So, this discovery of axion quasiparticles in a thin sheet of material could eventually lead to the discovery of axion particles that exist throughout the universe.