Room-temperature realization of macroscopic quantum phases is one of the significant pursuits in fundamental physics. The quantum spin Hall phase is a topological quantum phase that features a two-dimensional insulating bulk and a helical edge state.
In a new study, Princeton scientists reported novel quantum effects in a topological insulator at room temperature. This experiment is the first time these effects have been observed at room temperature. Temperatures near absolute zero, or -459 degrees Fahrenheit, are usually required to induce and observe quantum states in topological insulators (or -273 degrees Celsius).
This discovery opens up a new set of opportunities for the creation of effective quantum technologies, such as spin-based electronics, which have the potential to replace many existing electronic systems in favor of ones that use less energy.
M. Zahid Hasan, the Eugene Higgins Professor of Physics at Princeton University, who led the research, said, “The novel topological properties of matter have emerged as one of the most sought-after treasures in modern physics, both from a fundamental physics point of view and for finding potential applications in next-generation quantum engineering and nanotechnologies.”
“This work was enabled by multiple innovative experimental advances in our lab at Princeton.”
Topological insulators are the primary device element utilized to delve into the mysteries of quantum topology. This is a special gadget because the inside acts as an insulator, preventing the electrons from freely moving around and conducting electricity.
However, the device’s edges have free-moving electrons, indicating that they are conductive. Additionally, the electrons moving along the edges are not impeded by any flaws or deformations due to the unique characteristics of topology. By examining quantum electrical properties, this device has the potential to advance technology while also fostering a deeper knowledge of matter itself.
Hasan said, “Until now, however, there has been a major stumbling block in the quest to use the materials and devices for applications in functional devices. There is a lot of interest in topological materials, and people often talk about their great potential for practical applications. Still, these applications will likely remain unrealized until some macroscopic quantum topological effect can manifest at room temperature.”
This is due to the phenomenon known as “thermal noise,” which physicists define as a rise in temperature to the point where the atoms start to vibrate violently. This operation can collapse the quantum state by disrupting fragile quantum systems. Particularly with topological insulators, these higher temperatures lead to a situation in which the electrons on the surface of the insulator invade the interior, or “bulk,” of the insulator and induce the electrons there also to start conducting, diluting, or breaking the unique quantum effect.
This can be avoided by exposing such experiments to shallow temperatures, usually at or close to absolute zero. Atomic and subatomic particles stop vibrating at these shallow temperatures, making them easier to control. For many applications, creating and maintaining an ultra-cold environment is not feasible because doing so is expensive, large, and energy intensive.
Scientists here have come up with an innovative way to bypass this problem. They fabricated a new topological insulator from bismuth bromide (chemical formula α-Bi4Br4). It is an inorganic crystalline compound sometimes used for water treatment and chemical analyses.
Nana Shumiya, who earned her Ph.D. at Princeton, said, “This is just terrific that we found them without giant pressure or an ultra-high magnetic field, thus making the materials more accessible for developing next-generation quantum technology. I believe our discovery will significantly advance the quantum frontier.”
Hasan said, “The kagome lattice topological insulators can be designed to possess relativistic band crossings and strong electron-electron interactions. Both are essential for novel magnetism. Therefore, we realized that kagome magnets are a good system to search for topological magnet phases, as they are like the topological insulators we discovered and studied more than ten years ago.”
“A suitable atomic chemistry and structure design coupled to first-principles theory is the crucial step to make topological insulator’s speculative prediction realistic in a high-temperature setting. There are hundreds of topological materials, and we need intuition, experience, materials-specific calculations, and intense experimental efforts to find the right material for in-depth exploration. And that took us on a decade-long journey of investigating many bismuth-based materials.”
Hasan and his colleagues investigated the family of compounds called bismuth bromide in response to a proposal by collaborators and co-authors Fan Zhang and Yugui Yao to investigate a particular class of Weyl metals. The Weyl phenomenon, however, was not visible to the investigators in these materials. Instead, Hasan and his team found that the bismuth bromide insulator possesses characteristics that make it more desirable than topological insulators (Bi-Sb alloys) based on bismuth-antimony they had previously researched.
It has a sizeable insulating gap of over 200 meV (“milli electron volts”). This is large enough to overcome thermal noise but small enough so that it does not disrupt the spin-orbit coupling effect and band inversion topology.
Hasan said, “In this case, in our experiments, we found a balance between spin-orbit coupling effects and large band gap width. We found there is a ‘sweet spot’ where you can have a relatively large spin-orbit coupling to create a topological twist as well as raise the band gap without destroying it. It’s like a balance point for the bismuth-based materials we have been studying for a long time.”
When they could see what was happening in the experiment with sub-atomic resolution using a scanning tunneling microscope, a special tool that makes use of the phenomenon known as “quantum tunneling,” where electrons are directed between the sharp metallic, single-atom tip of the microscope and the sample, the scientists knew they had succeeded in their goal.
Hasan said, “For the first time, we demonstrated a class of bismuth-based topological materials that the topology survives up to room temperature. We are very confident of our result.”
“The researchers believe this breakthrough will open the door to future research possibilities and applications in quantum technologies.”
Shafayat Hossain, a postdoctoral research associate in Hasan’s lab and another co-first author of the study, said, “We believe this finding may be the starting point of future development in nanotechnology. There have been so many proposed possibilities in topological technology that await, and finding appropriate materials coupled with novel instrumentation is one of the keys for this.”
“Currently, the theoretical and experimental focus of the group is concentrated in two directions: First, we want to determine what other topological materials might operate at room temperature, and, importantly, provide other scientists the tools and novel instrumentation methods to identify materials that will operate at room and high temperatures.”
“Second, we want to continue to probe deeper into the quantum world now that this finding has made it possible to conduct experiments at higher temperatures.”
Hasan said, “These studies will require the development of another set of new instrumentations and techniques to harness these materials’ enormous potential fully. With our new instrumentation, I see a tremendous opportunity for further in-depth exploration of exotic and complex quantum phenomena, tracking finer details in macroscopic quantum states. Who knows what we will discover?”
“Our research is a real step forward in demonstrating the potential of topological materials for energy-saving applications. What we’ve done here with this experiment is plant a seed to encourage other scientists and engineers to dream big.”
Journal Reference:
- Nana Shumiya et al., Evidence of a room-temperature quantum spin Hall edge state in a higher-order topological insulator, Nature Materials (2022). DOI: 10.1038/s41563-022-01304-3