Scientists have been trying to develop room-temperature superconductors—materials that conduct electrons with zero resistance, and do so without cumbersome, energy-sucking supercooling—for more than three decades. Now researchers predict that a new material called stanene, composed of a one-atom-thick sheet of tin, could act much like a room-temperature superconductor, leading to faster, more efficient microchips.
Stanene is a type of topological insulator, a novel class of materials that have intrigued researchers for the past decade. While the interior of such a material is an electrical insulator, the outside edges and surfaces are electrically conductive. If exploited properly, this weird property of topological insulators can make it possible for electrons to flow without resistance.
To understand, an explanation of how electrons move in topological insulators is needed. Like tiny twirling bar magnets with north and south poles, electrons spin as they move around in a material. When electrons move around in the surfaces and edges of topological insulators, the direction of their spin becomes aligned with the direction of their flow. A consequence of this effect—known as the quantum spin Hall state—is that flowing electrons can't easily reverse direction. That is true even if they hit an impurity within the material—an event that in normal conductors causes electrons to scatter backwards and dissipate energy.
When electrons travel along the surface of a three-dimensional topological insulator, they generally can't bounce backwards, but they can still jostle each other sideways, wasting energy. But in two-dimensional topological insulators—surfaces that are just one atom thick—flowing electrons become restricted to a single lane, eliminating all interference. Recent experiments confirm that electrons can zip along the edges of flat topological insulators with 100 percent efficiency.
In the past decade, researchers have made topological insulators from compounds of electron-rich, heavy elements including mercury, bismuth, antimony, tellurium and selenium. None of them were perfect conductors of electricity at room temperature. Then Stanford University theoretical physicist Shoucheng Zhang and colleagues decided to investigate tin, a similarly electron-rich, heavy element. The team's calculations suggest that single-atom layers of tin are topological insulators where electrons flow perfectly at and above room temperature.
Adding fluorine atoms (yellow) to a single layer of tin atoms (gray) should allow a predicted new material, stanene, to conduct electricity perfectly along its edges (blue and red arrows) at temperatures up to 100 degrees Celsius. The first application for this stanene-fluorine combination could be in wiring that connects the many sections of a microprocessor, allowing electrons to flow as easily as cars on a freeway.
Image: Courtesy of Yong Xu/Tsinghua University; Greg Stewart/SLAC
"It's surprising that it can work at such a high temperature," Zhang says. "Scientists have looked for dissipationless transport of electricity for many years, but usually the systems we find only work under extreme conditions, either very low temperature or strong magnetic fields." The scientists detailed their findings online Sept. 27 in the journal Physical Review Letters.
"What's nice about this work is how they show you can have a single-element material that is a topological insulator," says physicist Kang Wang at the University of California, Los Angeles, who did not take part in this study. "That makes everything much simpler."
Although stanene and superconductors can both act like perfect conductors of electricity, Zhang emphasizes that stanene is not a superconductor. While the edges of stanene act as a highway for electrons, those electrons still encounter "contact resistance" when they move between the stanene and normal conductors. In a superconductor, in contrast, electrons travel in pairs, a phenomenon that can eliminate contact resistance. In other words, a normal conductor essentially acts like a superconductor when it is placed in contact with a superconductor.
Zhang says stanene and related materials could find use in wiring that connects the many parts of microchips, boosting their speed and lowering their power needs. "I hope stanene can replace silicon," Zhang says. "Tin is cheap, abundant, stable and environmentally friendly."
The central processing units (CPUs) of computers have been limited to speeds of about 3 gigahertz since 2005 because of the way energy dissipates in electronics, Wang explains. If they operated any faster, heat would overwhelm the devices. "Minimizing energy dissipation can improve all electronics," Wang says.
Future researchers could tweak stanene to enhance its performance, Zhang says. For instance, adding fluorine atoms could enable stanene to let electrons flow effortlessly at temperatures higher than 100 degrees Celsius.
These findings still need experimental confirmation, and stanene's atomically thin, delicate layers could prove challenging to manufacture. "However, the remarkable thing is that in the field of topological insulators, every theoretical prediction so far has come true," Zhang says.