Axion electrodynamics in a topological insulating crystal.

Researchers from the Laboratory for Quantum Magnetism working alongside international collaborators from Oxford, Dresden and Hamburg have identified a crystalline solid which satisfies the requirements to host an exotic axionic electrodynamic field. The work could make it possible to observe axion-like quasiparticles in the solid state, as well as laying the ground for a new type of dark matter detector.

Axions are a type of fundamental particle first proposed in 1977 to resolve the so-called strong CP problem in the Standard Model of particle physics. Axions have 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.

Recently, several theoretical studies have predicted that a quantized axion field that is absent from the usual Maxwell’s equations in media can occur within certain three-dimensional crystals, called axion insulators. The electronic spectrum of such a material is insulating within the bulk and on the surface due to an inversion of the electronic states relative to the atomic energy levels combined with magnetic order. To create the necessary condition for the axion field to exist, the magnetic order must satisfy stringent symmetry conditions.

The team, which included Dr Jian-Rui Soh, Professor Henrik M. Rønnow and Professor Frédéric Mila investigated a compound containing europium, indium and arsenic, with chemical formula EuIn2As2. They performed resonant x-ray scattering experiments at the Diamond Light Source, near Oxford, and at the PETRA-III facility in Hamburg, to study the magnetic pattern adopted by the europium electrons at low temperatures.

The experiments showed that magnets attached to the europium atoms self-organise into an unusual type of helical pattern, like a screw thread, which undergoes a scissor-like motion as the temperature increases. The team developed a theory that low energy vibrations of the helix cause the change in the structure with temperature. Importantly, the magnetic helix established in the experiments possesses the symmetries required for EuIn2As2 to be an axion insulator.