Bismuth crystals stacked to produce novel topological insulator

Spintronics refers to a suite of physical systems that may one day replace many electronic systems. To realize this generational leap, material components that confine electrons in one dimension are highly sought after. For the first time, researchers have now created such a material, known as a higher-order topological insulator, in the form of a special bismuth-based crystal.

For spintronic applications, a new kind of electronic material is required, and it's called a topological insulator. A topological insulator differs from a conductor, insulator or semiconductor because it's insulating throughout its bulk but conducting along its surface. And what it conducts is not the flow of electrons themselves, but a property of electrons known as their spin or angular momentum. This spin current, as it's known, could open up a new world of ultrahigh-speed and low-power electronic devices.

However, not all topological insulators are equal. Two kinds, so-called strong and weak, have already been created, but they have some drawbacks: as they conduct spin along their entire surface, the electrons present tend to scatter, which weakens their ability to convey a spin current. But since 2017, a third kind of topological insulator, called a higher-order topological insulator, has been theorized.

Now, for the first time, this third kind of topological insulator has been created by a team from the Institute for Solid State Physics at the University of Tokyo in Japan. The team reports its advance in a paper in Nature Materials.

"We created a higher-order topological insulator using the element bismuth," said Takeshi Kondo, an associate professor at the University of Tokyo. "It has the novel ability of being able to conduct a spin current along only its corner edges, essentially one-dimensional lines. As the spin current is bound to one dimension instead of two, the electrons do not scatter so the spin current remains stable."

To create this three-dimensional crystal, Kondo and his team stacked two-dimensional slices of crystal one atom thick in a certain way. For strong or weak topological insulators, crystal slices in the stack are all oriented the same way, like playing cards face down in a deck. But to create the higher-order topological insulator, the researchers alternated the orientation of the slices: the metaphorical playing cards were placed face up and then face down repeatedly throughout the stack. This subtle change in arrangement makes a huge change to the behavior of the resultant three-dimensional crystal.

The crystal layers in the stack are held together by a quantum mechanical force called the van der Waals force. This is one of the rare kinds of quantum phenomena that has a noticeable effect in daily life, as it is partly responsible for the way that powdered materials clump together and flow. In the crystal, it adheres the layers together.

"It was exciting to see that the topological properties appear and disappear depending only on the way the two-dimensional atomic sheets were stacked," said Kondo. "Such a degree of freedom in material design will bring new ideas, leading toward applications including fast and efficient spintronic devices, and things we have yet to envisage."

This story is adapted from material from the University of Tokyo, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.