Scientists observe axion quasiparticles in a lab for the first time, offering new insights into dark matter and future quantum technologies.
Northeastern University scientists and international collaborators have successfully created laboratory conditions that allowed them to observe axion quasiparticles for the first time, bringing researchers closer to understanding dark matter.
The research published this week in Nature represents a significant step in bridging the gap between theoretical physics and experimental proof, which can lead to both a better understanding of the universe and applications in future technology of magnetic memory.
The research — an effort that included more than a dozen organizations across five countries — included three Northeastern physicists: Arun Bansil, a university distinguished professor and director of the Quantum Materials and Sensing Institute; Kin Chung Fong, an associate professor of physics and electrical and computer engineering; and Barun Ghosh, a postdoctoral student.
“This study provides another exciting example of the very rich tapestry of quasiparticles that are harbored by quantum matter,” Bansil says. “It is clear that quantum materials will continue to offer us surprises long into the future to open new pathways for addressing pressing fundamental science questions as well as materials platforms for developing transformational new technologies.”
Everything that humans have discovered and observed in the universe — stars, planets, galaxies, gas and dust — makes up only about 5% of existing matter. Fong says that much of the universe’s composition remains a mystery, and dark matter is believed to account for the missing mass.
In 1978, physicists Frank Wilczek and Steven Weinberg independently theorized the existence of a new hypothetical elementary particle — axion — a possible component of dark matter.
“Since then, people have been hunting for it for a number of years,” Fong says.
The way he conceptualizes axions provides an intriguing picture of our place in the universe.
“What we believe right now is that axions actually are distributed across our universe. I think about it like a big wave that we are swimming inside,” he says. “Their congregation could prevent our galaxy from falling apart.”
Scientists have known for some time that our galaxy — the Milky Way — is spinning faster than expected. Dark matter could play a key role in holding the galaxy together by providing additional gravitational pull.
More recently, theorists proposed that excitations that mimic axions and known as the “dynamical axion quasiparticles” could exist under special conditions in certain Earth-based materials. They occur in a new class of lab-created materials — called antiferromagnetic topological insulators. The coherent oscillation of the magnetism in these materials, governed by their topological symmetry — a deep, built-in order in the material that doesn’t change even if it gets stretched or deformed — can simulate axion-like properties.
“It is fascinating to see the connections between the large-scale astrophysics and the nano-scale spin dynamics of electrons,” Fong says. “By studying the quasiparticles in the materials, we can have a glimpse into how the true axion may behave, and explore how its unique properties could be exploited for applications such as high-speed magnetic memories for computers.”
The new paper describes the right conditions that the researchers were able to create to observe the elusive axionic quasiparticles.
The research team used a laboratory-synthesized material called manganese bismuth telluride (MnBi₂Te₄).
Bismuth telluride is a gray, crystalline semiconductor that has been known for some time, Fong says. It is a thermoelectric material used for cooling and waste heat recovery. Manganese turns bismuth telluride into a magnetic topological insulator.
The experiment led by Su-Yang Xu’s group at Harvard University overcomes the experimental challenge to catch the dynamical axionic quasiparticles in action using ultrafast optics. The novel technique uses a pump to excite the axionic quasiparticles and probe them stroboscopically so fast that the light from this very word that you are reading wouldn’t have enough time to reach your eyes.
This discovery is significant because these quasiparticles behave like the theoretically predicted axion particles, which could help explain dark matter and solve fundamental problems in physics.
“Because their dynamics follow the same physics equations,” Fong says, “the real dark matter can excite those quasiparticles in these newly found materials. This provides a new avenue for us to search for axions in the universe.”
Ghosh, Bansil and his longtime collaborators Hsin Lin from Academia Sinica and Tay-Rong Chang from National Cheng Kung University in Taiwan carried out in-depth theoretical modeling and computations for interpreting the key experimental findings.
Fong and a number of colleagues from other institutions worked on calculations for estimating how sensitive a detector would need to be in order to detect a dark matter axion.
Beyond being a model of dark matter axions, the discovery also opens new doors for us to search for axions in the universe. If real axions interact with axionic quasiparticles, Fong says, they could drive the quasiparticles to emit detectable photons — that is, the emission of electromagnetic energy in discrete packages.
To turn this new concept to reality, the Department of Energy has recently selected Fong’s proposal to build a single-photon detector. In the future, this detector could be used in a telescope designed to detect axion.
“We are living in an exciting time of science,” Fong explains. “Astrophysicists, solid-state physicists and engineers are collaborating closely to develop materials that would enable quantum sensors to search for dark matter. This research connects deepest human endeavors toward understanding our universe as well as quantum technologies that could revolutionize computers and communications. We are proud to be in this frontier of quantum materials and sensing at Northeastern.”