Physicists show how to detect higher-order topological insulators

Just as a book cannot be judged by its cover, a material cannot always be judged by its surface. But, for a supposedly elusive class of materials, physicists have now shown that the surface previously thought to be “featureless” holds an unmistakable signature that could lead to the first definitive observation.

Higher-order topological insulators, or HOTIs, have attracted attention for their ability to conduct electricity along one-dimensional lines on their surfaces, but this property is quite difficult to distinguish experimentally from other effects. By instead studying the interior of these materials from a different perspective, a team of physicists identified a surface signature unique to HOTIs, capable of determining how light reflects off their surfaces.

As the team reports in the newspaper Natural communications, this property could be used to experimentally confirm the existence of such topological states in real materials.

“The bulk or interior properties of HOTIs and other topological insulators have been neglected for a long time, but it turns out that a lot of interesting things are happening there too,” said Barry Bradlyn, a physics professor at the University of Texas. Illinois. Urbana-Champaign and a co-leader of the project. “When we looked at the surfaces with a closer lens, they immediately stood out as being far from trivial or featureless.”

Topological insulators have long been known for their ability to carry electrical currents across their surfaces while having insulating interiors. HOTIs, however, would limit electrical conduction to a one-dimensional edge, or “hinge,” rather than the entire two-dimensional surface.

“Charles Kane, who discovered topological insulators, introduced a good analogy,” said Benjamin Wieder, a faculty member at the Institute of Theoretical Physics at Université Paris-Saclay and co-leader of the project. “We can think of standard topological insulators as Hershey’s Kisses. A conductive metal foil wrapped around an insulator that doesn’t conduct electricity, chocolate in this case, is a very good way to understand them. With HOTI , however, it’s like someone took the foil and crumpled it into a thin ring surrounding the chocolate.

While surface conductive states have been observed in standard topological insulators, resolving the hinge in HOTIs has proven exceptionally difficult. Bradlyn explained that this property can only exist in material samples with an unusually high degree of symmetry, meaning their crystal structures must be unrealistically perfect.

Instead, Bradlyn and his collaborators turned their attention from the hinge state inward, where electrons tend to “delocalize” from individual atoms and spread throughout the material. Unlike previous studies that treated all electrons the same, the researchers took into account differences in spin, a property of electrons that allows them to behave like miniature magnets.

“When we split the interior electrons into two possible spin states, high and low, we saw that each state leaves a unique surface signature,” said Kuan-Sen Lin, a graduate student in physics at the University of I. main author. “Even though the surface of a HOTI seems uninteresting, when you look at what each rotation does separately on the surface, an unmistakable new behavior appears that we hope will soon be measured experimentally.”

Because electrons with different spins behave like magnets, they react differently when an electrical voltage is applied to the material, causing the two spin states to accumulate on opposite sides. This accumulation can be detected by taking advantage of the magneto-optical Kerr effect, in which the polarization, or orientation of light, changes when it is reflected from the surface of a magnet. In the case of HOTIs, the researchers calculated the change in polarization of each spin state and found that it was exactly half the change that would result from an ordinary insulator.

“In the Kiss analogy, we might expect that because the foil was crumpled, the chocolate would be in direct contact with air,” said Gregory Fiete, a professor of physics at Northeastern University and author study correspondent. “With the spin-dependent surface behaviors we discovered, we can say that there is actually a transparent layer that keeps the chocolate separate from the rest of the supermarket.”

Drawing on first-principles calculations with the specialized theoretical toolbox the researchers developed for this study, they identified the metallic bismuth bromide as a very strong candidate for observing this effect. They are currently working with Fahad Mahmood, professor of physics at the University of I., and Daniel Shoemaker, professor of materials science and engineering at the University of I., to design and carry out the experiments proposed in this study.

“The properties of HOTIs that we identified here would be very useful in quantum computing and spintronic devices, but we need to see them experimentally first,” Bradlyn said. Wieder added: “We hope our work will show that the interiors and surfaces of topological materials still harbor many mysterious and advantageous features if you know to look for them.”

The first principle calculations on bismuth bromide were carried out by Zhaopeng Guo and Zhijun Wang of the Chinese Academy of Sciences. Additional computational support was provided by Jeremey Blackburn of Binghamton University. Giandomenico Palumbo of the Dublin Institute for Advanced Studies and Yoonseok Hwang of the University of I. also contributed to this work.