Physicists have observed axion quasiparticles for the first time in a two-dimensional quantum material. As well as having applications in materials science, the discovery could aid the search for fundamental axions, which are a promising (but so far hypothetical) candidate for the unseen dark matter pervading our universe.
Theorists first proposed axions in the 1970s as a way of solving a puzzle involving the strong nuclear force and charge-parity (CP) symmetry. In systems that obey this symmetry, the laws of physics are the same for a particle and the spatial mirror image of its oppositely charged antiparticle. Weak interactions are known to violate CP symmetry, and the theory of quantum chromodynamics (QCD) allows strong interactions to do so, too. However, no-one has ever seen evidence of this happening, and the so-called “strong CP problem” remains unresolved.
More recently, the axion has attracted attention as a potential constituent of dark matter – the mysterious substance that appears to make up more than 85% of matter in the universe. Axions are an attractive dark matter candidate because while they do have mass, and theory predicts that the Big Bang should have generated them in large numbers, they are much less massive than electrons, and they carry no charge. This combination means that axions interact only very weakly with matter and electromagnetic radiation – exactly the behaviour we expect to see from dark matter.
Despite many searches, though, axions have never been detected directly. Now, however, a team of physicists led by Jianxiang Qiu of Harvard University has proposed a new detection strategy based on quasiparticles that are axions’ condensed-matter analogue. According to Qiu and colleagues, these quasiparticle axions, as they are known, could serve as axion “simulators”, and might offer a route to detecting dark matter in quantum materials.
Topological antiferromagnet
To detect axion quasiparticles, the Harvard team constructed gated electronic devices made from several two-dimensional layers of manganese bismuth telluride (MnBi2Te4). This material is a rare example of a topological antiferromagnet – that is, a material that is insulating in its bulk while conducting electricity on its surface, and that has magnetic moments that point in opposite directions. These properties allow quasiparticles known as magnons (collective oscillations of spin magnetic moments) to appear in and travel through the MnBi2Te4. Two types of magnon mode are possible: one in which the spins oscillate in sync; and another in which they are out of phase.
Qiu and colleagues applied a static magnetic field across the plane of their MnBi2Te4 sheets and bombarded the devices with sub-picosecond light pulses from a laser. This technique, known as ultrafast pump-probe spectroscopy, allowed them to observe the 44 GHz coherent oscillation of the so-called condensed-matter field. This field is the CP-violating term in QCD, and it is proportional to a material’s magnetoelectric coupling constant. “This is uniquely enabled by the out-of-phase magnon in this topological material,” explains Qiu. “Such coherent oscillations are the smoking-gun evidence for the axion quasiparticle and it is the combination of topology and magnetism in MnBi2Te4 that gives rise to it.”
A laboratory for axion studies
Now that they have detected axion quasiparticles, Qiu and colleagues say their next step will be to do experiments that involve hybridizing them with particles such as photons. Such experiments would create a new type of “axion-polariton” that would couple to a magnetic field in a unique way – something that could be useful for applications in ultrafast antiferromagnetic spintronics, in which spin-polarized currents can be controlled with an electric field.
The axion quasiparticle could also be used to build an axion dark matter detector. According to the team’s estimates, the detection frequency for the quasiparticle is in the milli-electronvolt (meV) range. While several theories for the axion predict that it could have a mass in this range, most existing laboratory detectors and astrophysical observations search for masses outside this window.
“The main technical barrier to building such a detector would be grow high-quality large crystals of MnBi2Te4 to maximize sensitivity,” Qiu tells Physics World. “In contrast to other high-energy experiments, such a detector would not require expensive accelerators or giant magnets, but it will require extensive materials engineering.”
The research is described in Nature.
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