About a decade ago, physicists began identifying and probing materials that suffer something of an identity crisis. On the surface of these crystals, electrons flow and form currents, like in a conductor. But in the interior, electrons are pinned and unable to move, like in an insulator. That surface behavior is protected by the crystal structure itself. Materials that exhibit this phenomenon are called topological insulators. Since theoreticians predicted in 2005 which materials might show this behavior, physicists have found a wide variety of materials with surprising electronic behavior on their edges.
Now in a new paper in Nature Physics, researchers report on a new family of topological insulators (TIs) in “heavy electron” materials, so called because their electrons behave as if they’re hundreds of times more massive than they actually are. The new work focuses on materials known as Kondo insulators, which behave semi-metallic at room temperature but become insulators when cooled to near absolute zero. Previous studies on topological Kondo insulators (TKIs) have focused on crystals with simple crystal symmetries. However, the new work models a TKI whose unusual symmetries produce conducting surface states that “twist” in unusual ways—reminiscent of a Möbius strip whose twist gives the shape only one boundary.
In 2010, physicists led by theorist Piers Coleman at Rutgers University, in New Jersey, theoretically predicted that Kondo insulators would be good candidates for topological insulators. Their prediction proved correct: the first Topological Kondo Insulator (TKI) was verified experimentally, in samarium hexaboride, by other physicists in 2014. This was important because previous experiments have shown many TI materials, despite the theoretical predictions, still retain some conductivity in their interior; samarium hexaboride had none.
That verification, says Coleman, led them to wonder: What other Kondo insulators might be TKIs? Past work suggested that topological insulators often hide in plain sight, in materials with strange but well-documented behaviors. Mercury telluride and bismuth antimony, the first two materials shown to be TIs, occur naturally, and physicists have been documenting their curious properties since the 1960s.
Two researchers in Coleman’s group, Po-Yao Chang and Onur Erten (now at the Max Planck Institute in Dresden, Germany), examined cerium nickel tin, a semi-metal material known to be a Kondo insulator. They knew that previous studies had identified unusual symmetries in the crystal structure of the material, including glide translations and screw rotations. These are called “nonsymmorophic” symmetries, and the physicists suspected they might result in conductive behavior on the surface. They were right: In the recent paper, they not only report the new TKI, but present a model showing how those complicated symmetries in the crystal arrangement lead to twisted, “Möbius” electron behavior.
“It’s a new type of surface state,” says Chang. Following previous work on Möbius structure, the researchers named the new material a Möbius Kondo Insulator. It may be the first of many discovered with twisted edge states—a whole new class of exotic materials.
Physicist Dae-Jeong Kim, at the University of California, Irvine, wasn’t surprised to see the twisted surface states identified in cerium nickel tin. But Kim, who was part of the team that identified samarium hexaboride as a TKI in 2014, says the new work shows how new experimental investigations can reveal extraordinary properties in these materials. “It may well lead to another gold rush to explore the strongly correlated topological insulator systems,” says Kim, adding that he wouldn’t be surprised if future studies turned up even stranger behaviors in TKIs.
The hunt for materials with surprising interactions between electron behavior and topology is a fertile area, says Coleman—in part because unusual properties of TIs make them appealing for new electronic technologies. But perhaps more importantly, for scientists who probe quantum mysteries, these materials are powerful tools for asking big, fundamental questions about electron behavior. “They will probably change our understanding of quantum mechanics,” says Coleman.