Scientists observe complex tunable magnetism in a topological material
Scientists at the US Department of Energy’s Ames Laboratory have observed a novel helical magnetic order in the topological compound EuIn2As2 that supports an exotic electrical conduction that can be tuned by a magnetic field. The discovery has a significant impact on basic research on functional topological properties and could one day find use in a number of advanced technology applications.
Topological materials appeared in the natural sciences about fifteen years ago, decades after their existence was theorized. Described as “topological” because their electronic bulk bands are “knotted together”, the surfaces of the topological insulators loosen the knot and become metallic. Researchers at the Ames Laboratory Center for Advancement of Topological Semimetals (CATS) are trying to discover, understand and control the extraordinary conductivity properties of these materials.
Much of modern technology relies on crystalline materials, which are solids made up of a repeating (periodic) arrangement of atoms that form a lattice. Due to the periodicity, the grid looks the same after certain symmetry operations such as translation, specific rotations, mirrors and / or inversion. The presence or absence of these symmetries affects the electronic tape topology and the electronic surface conduction. The magnetic order can modify the symmetries of the material and provides an additional means of controlling the topological state.
Working with scientists from the Spallation Neutron Source at Oak Ridge National Laboratory, McGill University, and the University of Missouri Research Reactor Center, the CATS team discovered the existence of a helical magnetic order of low symmetry in EuIn2As2 that supports a very desirable topological state called a Axion isolator. This state has similarities with the axion particle in quantum chromodynamics, which is a candidate component of dark matter. In solid materials it offers a remarkable parallel coupling between magnetic and electrical properties.
In the presence of the complex helical magnetic order of EuIn2As2, the axion state leads to topological features in the electronic surface spectrum called Dirac cones. When a Dirac cone occurs on a surface of the material that is penetrated by a fundamental axis of magnetic order, the cone has no energy gap and the surface exhibits unresistive conduction tied to the orientation of the electronic spin. The other surfaces have split Dirac cones and support half-integer quantized electrical conduction. The researchers predict that by applying a relatively moderate magnetic field, the surface will support what type of Dirac cone, which will allow the surface conduction to be tuned.
The ability to switch between surface states through a magnetic field provides an experimental way to study the unique properties of its topological states. This tunability is also promising for technologies such as high-precision sensors, resistance-free nanowires, magnetic storage media and quantum computers. Future studies will address bulk crystals with the application of a magnetic field and synthesize and examine nanoscale thin films to pave the way for technological applications.
The paper “Magnetically Crystalline Symmetrical Protected Axion Electrodynamics and Field-Tunable, Unfixed Dirac Cones in EuIn2As2” was written by SXM Riberolles, TV Trevisan, B. Kuthanazhi, TW Heitmann, F. Ye, DC Johnston and SL Bud’ko DH Ryan, PC Canfield , A. Kreyssig, A. Vishwanath, RJ McQueeney, L. -L. Wang, PP Orth & amp; BG Ueland; and published in Nature Communications.
This research was supported by the Center for the Advancement of Topological Semimetals, an Energy Frontier Research Center funded by the U.S. Department of Energy and directed by the Ames Laboratory. This research used resources at Spallation Neutron Source, a user facility of the DOE Office of Science operated by Oak Ridge National Laboratory. This research used resources at the Missouri University Research Reactor.
Ames Laboratory is a US Department of Energy national laboratory operated by Iowa State University. Ames Laboratory develops innovative materials, technologies and energy solutions. We use our specialist knowledge, our unique skills and our interdisciplinary collaboration to solve global problems.
Ames Laboratory is supported by the US Department of Energy’s Office of Science. The Office of Science is the biggest proponent of basic science in the United States, working to address some of the most pressing challenges of our time. Further information can be found at https://energy.gov/science.
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