The building blocks for exploring exotic new states of matter

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Science

Topological insulators act as electrical insulators inside but conduct electricity along their surfaces. Researchers are studying the exotic behavior of some of these insulators by using an external magnetic field to force the ion spins in a topological insulator to be parallel to each other. This process is known as time reversal symmetry breaking. Now, a research team has created an intrinsic ferromagnetic topological insulator. This means that the time reversal symmetry is broken without applying a magnetic field. The team used a combination of synthesis, characterization and theory tools to confirm the structure and properties of novel magnetic topological materials. In the process, they discovered an exotic MnBi8Te13 axion insulator.

The impact

Researchers can use magnetic topological materials to realize exotic forms of matter not seen in other types of materials. Scientists believe that the phenomena exhibited by these materials could help advance quantum technology and increase the energy efficiency of future electronic devices. Researchers believe that an inherently ferromagnetic topological insulator, rather than acquiring its properties by adding a small number of magnetic atoms, is ideal for studying new topological behaviors. Indeed, no external magnetic field is necessary to study the properties of the material. It also means that the magnetism of the material is more evenly distributed. However, scientists have already faced challenges in creating this type of material. This new material is made up of layers of manganese, bismuth and tellurium atoms. This could provide opportunities to explore new phases of matter and develop new technologies. It also helps researchers investigate fundamental scientific questions about quantum materials.

Summary

The research team, led by scientists from the University of California, Los Angeles, developed the intrinsic ferromagnetic topological insulator by fabricating a compound with alternating layers of MnBi2Te4 and Bi2Te3, held together by weak interlayer attractive forces. between molecules. Scientists have recently discovered that MnBi2Te4 is a naturally magnetic topological material. However, when layers of magnetic MnBi2Te4 are directly stacked on top of each other, the magnetic moments in neighboring layers point in opposite directions, making the material antiferromagnetic as a whole – losing topological aspects of properties important for technologies. The researchers solved this problem by creating a new compound with three non-magnetic layers of Bi2Te3 between layers of MnBi2Te4, which when combined create MnBi8Te13. This material design increases the distance between MnBi2Te4 layers, which successfully eliminates the antiferromagnetic effect, leading to long-range ferromagnetism of less than 10.5 K with strong coupling between magnetism and charge carriers.

Important aspects of this research were neutron scattering experiments using the DEMAND instrument at the High Flux Isotope Reactor (HFIR) which highlighted the way the atoms are arranged in the MnBi8Te13 material and confirmed its ferromagnetic state. Because neutrons have their own magnetic moment, they can be used to determine the magnetic structure inside a material. The scientists also used angular-resolved photoemission spectroscopy experiments at the Stanford synchrotron radiation light source, a Department of Energy user facility, and first principles, density functional theory calculations to study the electronic and topological state of material. By combining the evaluations of all these methods, the researchers were able to validate the ferromagnetic and topological properties consistent with an axion insulator with large surface hybridization gaps and a non-trivial Chern number.

Funding

The research was supported by DOE Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division and Scientific User Facilities Division, National Science Foundation, Indian Institute of Technology Kanpur, Council for Scientific and Industrial Research in India , Taiwan Ministry of Science and Technology, National Cheng Kung University, National Center for Theoretical Science of Taiwan, and Taiwan Ministry of Education. The research was performed at three DOE Office of Science user facilities: the High Flux Isotope Reactor at Oak Ridge National Laboratory, the Stanford Synchrotron Radiation Light Source at the SLAC Accelerator National Laboratory, and the Lawrence Berkeley National Laboratory’s National Energy Research Scientific Computing.

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