In particle physics, the Standard Model is a theoretical framework that describes the building blocks of matter and the forces that govern their interactions. Incorporating three of the four fundamental forces (electromagnetic, weak nuclear and strong nuclear), the Standard Model is a highly successful and extensively tested theory that provides a comprehensive explanation of the behaviour of particles at the subatomic level. However, it does not provide a complete theory of the Universe. A long-standing problem with the Standard Model is that the theoretical equations describing strong interactions suggest that CP symmetry should be violated in a way that would lead to observable effects that have not been seen in experiments. Axions are one potential solution to this problem. First proposed in 1977, these hypothetical particles have since also emerged as one of the prime candidates for the dark matter required to account for the missing mass in the Universe. So far, however, axions have yet to be observed in nature. Recent theoretical studies predict that a quantised axion field can occur within certain three-dimensional crystals, called axion insulators. In work recently published in Nature Communications, an international team of researchers used resonant elastic X-ray scattering (REXS) to establish that EuIn2As2 has the necessary characteristics to realise axion electrodynamics. Their results could make it possible to observe axion-like quasiparticles in the solid state and lay the groundwork for a new type of dark matter detector.
Axions are hypothetical elementary particles that were originally proposed as a solution to a problem in the theory of strong interactions in particle physics, specifically related to the violation of CP symmetry. CP symmetry combines the concepts of charge conjugation (C), which involves swapping particles with their corresponding antiparticles, and parity (P), which involves flipping the spatial coordinates. The theoretical equations describing strong interactions in the Standard Model suggest that CP symmetry should be violated in a way that would lead to observable effects (like an electric dipole moment in the neutron) that have not been seen in experiments.
In the late 1970s, physicists Roberto Peccei and Helen Quinn postulated a solution to the strong CP problem, introducing a new particle (the axion) that would interact very weakly and thus avoid the experimental constraints. Despite decades of experimental searches, axions have not been directly observed. However, their existence is still a topic of active research, and various experiments and techniques are being developed to detect axions or to indirectly infer their presence based on their potential effects on various physical phenomena.
The properties of magnetic topological insulators and semimetals are strongly influenced by the coupling between magnetic spin configurations and non-trivial electronic topology. Some of these crystalline solids have types of antiferromagnetic order predicted to realise axion electrodynamics. Researchers from Oxford's Department of Physics, working alongside international collaborators, investigated the highly unusual helimagnetic phases in EuIn2As2, a potential candidate for an axion insulator. The magnetic order must satisfy stringent symmetry conditions to create the necessary condition for the axion field to exist.
The team used Resonant X-ray Scattering (REXS) experiments at Diamond's I16 beamline to determine the ground state magnetic order of the Eu spins at low temperatures, and similar experiments at PETRA-III in Hamburg to investigate its evolution in a magnetic field.
REXS provides a unique probe to study complex ordering phenomena in magnetic, quantum and functional materials. The combination of a small beam size, high resolution, and the ability to control and analyse the beam polarisation before and after the scattering process makes REXS unique in its ability to unravel and understand - in one measurement - spatial modulation of spin, charge, and orbital configuration.
Their results showed that the Eu spins self-organise into an unusual helical pattern that undergoes a scissor-like motion as the temperature increases. This magnetic helix has the symmetries required for EuIn2As2 to be an axion insulator. The team has also developed a model that explains the temperature-dependent changes in the magnetic order.
Their work illustrates how topological materials can be used as a laboratory with which to study unsolved puzzles about the Universe, and furthers the prospects for advanced technological applications that could benefit from the same exotic physics.
To find out more about the I16 beamline or discuss potential applications, please contact Principal Beamline Scientist Alessandro Bombardi: firstname.lastname@example.org.
Soh JR et al. Understanding unconventional magnetic order in a candidate axion insulator by resonant elastic x-ray scattering. Nature Communications 14, 3387 (2023). DOI:10.1038/s41467-023-39138-5.
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