Scientists observe complex tunable magnetism linked to electrical conduction in topological material


Figure 1. The complex helical magnetic order in EuIn2As2 (right) supports an axion insulator state with gap-free Dirac cones (left) appearing on surfaces penetrated by and perpendicular to the blue arrows of the magnetic structure. These surfaces support chiral conduction without resistance. Other surfaces are spaced apart and exhibit half-integer anomalous quantum Hall conduction at the edges of the sample. Credit: Ames Laboratory

Scientists at the US Department of Energy’s Ames Laboratory observed a new helical magnetic order in the topological compound EuIn2As2 which supports exotic electrical conduction tunable by a magnetic field. The discovery has important implications for basic research on functional topological properties and could one day be used in a number of advanced technological applications.

Topological materials burst into the physical sciences about fifteen years ago, decades after their existence was theorized. Called “topological” because their loose electronic bands are “knotted” together, the surfaces of topological insulators “untie the knot” and become metallic. Researchers at the Center for the Advancement of Topological Semi-Metals (CATS) at the Ames laboratory seek to discover, understand and control the exceptional conduction properties of these materials.

Much of modern technology relies on crystalline materials, which are solids made up of a repeated (periodic) arrangement of atoms that form a lattice. Due to the periodicity, the grating looks the same after certain symmetry operations such as translation, specific rotations, mirror and / or inversion. The existence or absence of these symmetries affects the topology of the electronic bands and the surface electronic conduction. The magnetic order can alter the symmetries presented by the material, providing an additional means of controlling the topological state.

Working with spallation neutron source scientists at the Oak Ridge National Laboratory, McGill University and the University of Missouri Reactor Research Center, the CATS team discovered the existence of ‘a low symmetry helical magnetic order in EuIn.2As2 which supports a highly sought-after topological state called an axionic insulator. This state shares similarities with the axion particle in quantum chromodynamics which is a candidate component of dark matter. In solid state materials, it provides 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 surface electronic spectrum called Dirac cones. When a Dirac cone appears on a surface of the material crossed by a fundamental axis of the magnetic order, the cone has no energy hole and the surface presents a conduction without resistance linked to the orientation of the electronic spin . The other surfaces have spread Dirac cones and support half-integer quantized electrical conduction. The researchers predict that applying a relatively mild magnetic field changes which surfaces support which type of Dirac cone, allowing surface conduction to be adjusted.

The ability to switch between surface states by a magnetic field provides an experimental way to examine the unique properties of its topological states. This tunability also holds promise for technologies such as high-precision sensors, resistance-free nanowires, magnetic storage media, and quantum computers. Future studies will examine loose crystals while applying a magnetic field and synthesize and study thin films at the nanoscale to pave the way for technological applications.

The article “Electrodynamics of axions protected by magnetic crystalline symmetry and unpinned Dirac cones tunable by field in EuIn2As2, is published in Nature Communication.

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More information:
SXM Riberolles et al. Electrodynamics of magnetic axions protected by crystal symmetry and field-tunable unpinned Dirac cones in EuIn2As2, Nature Communication (2021). DOI: 10.1038 / s41467-021-21154-y

Provided by the Ames laboratory

Quote: Scientists Observe Complex Tunable Magnetism Linked to Electrical Conduction in Topological Material (2021, March 22) retrieved October 7, 2021 from -tied.html

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