Diamond Annual Review 2023/24

35 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 3 / 2 4 Can synchrotron studies uncover the elusive axion? 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) at the I16 beamline to establish that EuIn2As2 has the necessary characteristics to realise axion electrodynamics. 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. Soh, J.R. et al . DOI: 10.1038/s41467-023-39138-5​ RIXS shows flat-band stoner excitations in a kagome semimetal Topological materials (including topological insulators, Dirac and Weyl semimetals and skyrmions) are a hot topic in science at the moment. A gold rush of sorts is underway, to discover and investigate the exotic physical properties of these materials, which could be the key that unlocks next- generation energy-efficient electronic devices and quantum computing. In some materials, geometrical confinement of electrons can give rise to electronic correlations that manifest as dispersionless ‘flat’ bands. These flat bands are of particular interest, as they can result in unconventional ferromagnetic and transport behaviour such as unconventional superconductivity, Mott-insulator transition and the fractional quantum Hall effect. However, there have been few characterisations of flat bands and their magnetism. In work recently published in Nature Communications, scientists from Diamond’s I21 beamline used high energy-resolution Co L 3- edge Resonant Inelastic X-ray Scattering (RIXS) to investigate the ferromagnetic Kagome semimetal Co3Sn2S2, reporting the first observation of flat-band Stoner excitations in this material. Their results also demonstrate that RIXS can clarify the magnon-Stoner interactions in itinerant correlated flat band systems. Nag, A. et al. DOI:10.1038/s41467-022-34933-y Figure: Illustrations of the study findings. Figure: The Kagome lattice. The schematic flat electronic bands and the corresponded well-defined Stoner excitations.