Publications
Understanding unconventional magnetic order in a candidate axion insulator by resonant elastic x-ray scattering
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.
Neutron scattering of local non-centrosymmetric magnetoelectric multipoles
The fundamental interaction between the neutron dipolar field and the magnetization density surrounding the scattering ion, lies at the heart of magnetic neutron diffraction. However, if the ion resides in an environment which breaks both time and spatial inversion symmetry, the current formalism for magnetic diffraction does not fully account for all the possible scattering mechanisms arising from the asymmetry of the magnetization density cloud of the scatterer.
In our work, we have (1) extended the theory of magnetic neutron diffraction to include these effects. Drawing analogies from the magneto-electric (ME) phenomena and standard magnetic neutron diffraction, (2) developed a framework to calculate the associated ME form factor, size of the ME multipoles and the ME propagation vector from density functional theory (DFT) calculations.
Furthermore, we have identified several material systems, which can not only host these ions but also display an ordered arrangement of these magneto-electric multipoles. Alongside our DFT calculation of the corrections to the scattering amplitudes and form factor of these multipoles, we used spherical neutron polarimetry to (3) provide evidence for the interactions between neutrons and the long-ranged order of these magneto-electric multipoles in CuO.
Discovery of an ideal Weyl semimetal
Weyl semimetals exhibit exceptional electronic transport due to the presence of topological band crossings called Weyl nodes. The nodes come in pairs with opposite chirality, but their number and location in momentum space is otherwise material-specific. Together with colleagues in the Rudolf Peierls Centre for Theoretical Physics, Oxford, and a team of international collaborators, we have found that the layered intermetallic EuCd2As2 in a magnetic field is what Bernevig has termed the hydrogen atom of a Weyl semimetal, i.e. one with a single pair of Weyl nodes at the Fermi level and without overlapping electron bands. The discovery opens the door to exploration of a wide range of exotic physics predicted for Weyl fermions in the solid state.
Magnetic structure and excitations of YbMnBi2
One of the major themes in solid-state physics is the realization of exotic types of relativistic electrons that travel at speeds much slower than the speed of light. These electrons live in crystals with very specific structural properties, one such material being the Weyl semimetal. The compound YbMnBi2 has been proposed as a candidate if it possesses a very specific type of magnetic order. In this work, we establish conclusively that YbMnBi2 is not a Weyl semimetal.