Topological magnet exhibits exotic quantum effects

Technology News | July 23, 2020

By Rich Pell

Materials & processes Power Management Test and Measurement

Working with an international team of scientists, the researchers say their findings provide insights into a 30-year-old theory of how electrons spontaneously quantize and demonstrate a proof-of-principle method to discover new topological magnets. Such quantum magnets hold promise for dissipationless current, high storage capacity, and future green technologies.

Topology is a branch of theoretical mathematics that is already known to play a powerful role in dictating the behavior of electrons in crystals. Topological materials can contain massless particles in the form of light, or photons. In a topological crystal, say the researchers, the electrons often behave like slowed-down light yet, unlike light, carry electrical charge.

Topological phases have been intensely studied in science and engineering and Many new classes of quantum materials with topological electronic structures have been found, including topological insulators and Weyl semimetals. However, say the researchers, while some of the most exciting theoretical ideas require magnetism, most materials explored have been nonmagnetic and show no quantization, leaving many tantalizing possibilities unfulfilled.

“The discovery of a magnetic topological material with quantized behavior,” says M. Zahid Hasan, the Eugene Higgins Professor of Physics at Princeton University, who led the research team, “is a major step forward that could unlock new horizons in harnessing quantum topology for future fundamental physics and next-generation device research.”

The researchers had been searching for a topological magnetic quantum state that may also operate at room temperature. Recently, they found a materials solution in a kagome lattice – a lattice structure found in many natural minerals’ molecular arrangements exhibiting novel physical properties – magnet that is capable of operating at room temperature, which also exhibits the much desired quantization.

“The kagome lattice can be designed to possess relativistic band crossings and strong electron-electron interactions,” says Hasan. “Both are essential for novel magnetism. Therefore, we realized that kagome magnets are a promising system in which to search for topological magnet phases as they are like the topological insulators that we [have] studied before.”

It took several years of intense research on several families of topological magnets, say the researchers, before they gradually realized that a material made of the elements terbium, magnesium, and tin (TbMn₆Sn₆) had the ideal crystal structure with chemically pristine, quantum mechanical properties and spatially segregated kagome lattice layers. Moreover, it uniquely features a strong out-of-plane magnetization.

The researchers synthesized this ideal kagome magnet at the large single crystal level and used scanning tunneling microscopy – which is capable of probing the electronic and spin wavefunctions of a material at the sub-atomic scale with sub-millivolt energy resolution – to confirm that the crystals were topological and, more important, featured the desired exotic quantum magnetic state.

“The first surprise was that the magnetic kagome lattice in this material is super clean in our scanning tunneling microscopy,” says Songtian Sonia Zhang, a co-author of a study on the work who earned her Ph.D. at Princeton earlier this year. “The experimental visualization of such a defect-free magnetic kagome lattice offers an unprecedented opportunity to explore its intrinsic topological quantum properties.”

When the researchers turned on a magnetic field, they found that the electronic states of the kagome lattice modulate dramatically, forming quantized energy levels in a way that is consistent with Dirac topology. By gradually raising the magnetic field to 9 Tesla – hundreds of thousands of times higher than the earth’s magnetic field – they systematically mapped out the complete quantization of this magnet.

“It is extremely rare – there has not been one found yet – to find a topological magnetic system featuring the quantized diagram,” says Nana Shumiya, a graduate student and co-author of the study. “It requires a nearly defect-free magnetic material design, fine-tuned theory, and cutting-edge spectroscopic measurements.”

The quantized diagram that the team measured provides precise information revealing that the electronic phase matches a variant of the topological Haldane model – a conceptual basis for theoretical and experimental research exploring topological insulators and superconductors. It confirms that the crystal features the characteristics expected by the theory for topological magnets.

Now, say the researchers, their theoretical and experimental focus is shifting to the dozens of compounds with similar structures to TbMn₆Sn₆ that host kagome lattices with a variety of magnetic structures, each with its individual quantum topology.

“This is like discovering water in an exoplanet,” says Hasan. “It opens up a new frontier of topological quantum matter research our laboratory at Princeton has been optimized for.”

For more, see “Quantum-limit Chern topological magnetism in TbMn₆Sn₆.”