Nature Materials cover First observation of quantum spin Hall edge states at room temperature at NIT
For the first time, physicists have observed a new quantum effect in a topological insulator at room temperature: the progress was made by Yugui Yao's team at the School of Physics, Beijing Institute of Technology, in collaboration with M. Zahid Hasan's team at Princeton University while exploring a topological material based on the element bismuth (Bi). The related results are titled "Evidence for room-temperature quantum spin Hall edge states in higher-order topological insulators" [1] and published as the cover article of the October issue of Nature Materials.
01Spin-edge states of topological insulators: a much-needed study
A topological insulator is a material that behaves as an insulator in its interior, but contains a protected conducting state on its surface. Two-dimensional (2D) topological insulators are characterized by a time-reversal symmetry-protected spin-edge state in the insulator gap and accordingly exhibit the quantum spin Hall effect. The spin-edge states are characterized by dissipation-free electron channels along the edges of the sample, which is of great interest in energy-saving technologies and quantum information science.
Among the topological insulator candidates, Bi4Br4 has a van der Waals bond-like layer structure and is believed to have a large insulating gap and weak interlayer coupling; therefore, a single layer of Bi4Br4 has the potential to realize high-temperature quantum spin Hall states in a freestanding and bulk environment.
(a) Schematic diagram of the quantum spin Hall edge state: counter-propagating spin edge state with spin-up (red) and spin-down (blue) dissipation-free channels in real space (left); the same topological edge state with red and blue bands in surface momentum space (cartoon view) projected (right). The edge state originates from the body band inversion, which is outlined by the orange and light blue bands. b) Three-dimensional crystal structure of α-Bi4Br4, top view of the monolayer (bottom right) and side view of the block (top right). c) Scanning transmission electron microscopy image from the side, showing interatomic layer AB stacking (AB stacking). The light blue curve is the difference spectrum taken at different locations on the surface, and the dark blue curve indicates the average spectrum.
However, there has been a major stumbling block in the quest to apply the material and device to functional devices. "There is a lot of interest in topological materials, and people often talk about their great potential for practical applications," said M. Zahid Hasan, a professor at Princeton University and corresponding author of the paper [2], "but until some macroscopic quantum topological effects can be manifested at room temperature until they can be demonstrated, these applications may remain elusive."
This is because ambient or high temperatures produce what physicists call "thermal noise," which is defined as a rise in temperature that causes atoms to start vibrating violently; this behavior disrupts delicate quantum systems and thus collapses quantum states. In particular, in topological insulators, these higher temperatures can create a situation where electrons on the surface of the insulator invade the interior of the insulator and cause the electrons there to start conducting as well, which dilutes or destroys the special quantum effects.
A way to get around this is to subject such experiments to exceptionally low temperatures, usually at or near absolute zero. At these incredibly low temperatures, atoms and subatomic particles stop vibrating and are therefore much easier to manipulate. However, creating and maintaining an ultracold environment is impractical for many applications: it is costly, bulky, and requires significant energy consumption.
Therefore, experimental studies in space with atomic-level spatial resolution, magnetic field tunability, and temperature control of the edge-state properties of topological insulators are highly desirable.
02Topological insulators at room temperature to advance the quantum frontier
Recently, Hasan's team developed an innovative approach: based on their experience with topological materials and in collaboration with many collaborators, they fabricated a new topological insulator made of bismuth bromide (chemical formula α-Bi4Br4), an inorganic crystalline compound sometimes used in water treatment and chemical analysis.
"It's just so exciting that we've discovered them without huge pressures or ultra-high magnetic fields, making it easier to use these materials for developing the next generation of quantum technologies." Nana Shumiya, Ph.D., a postdoctoral research associate in electrical and computer engineering at Princeton University and one of the paper's three co-first authors, said, "I believe our discovery will significantly advance the quantum frontier."
The discovery has its roots in the workings of the quantum Hall effect, a topological effect.
After discovering the first example of a three-dimensional topological insulator in 2007, Hasan's team had been searching for a topological quantum state that might also work at room temperature; after a decade, they finally found a material solution to the Hall conjecture in a kagome lattice that can work at room temperature and that also exhibits the required quantumization.
"Gowie lattice topological insulators can be designed to have relativistic band crossing and strong electron-electron interactions." Both are crucial for novel types of magnetism," Hasan said. Therefore, we realized that the Govy lattice is a promising system in which to look for topological magnet phases, because they are like the topological insulators we discovered and studied more than a decade ago."
"Proper atomic chemistry and structural design coupled with first-principles theory are key steps in making speculative predictions of topological insulators a reality in high-temperature environments," Hasan said. "There are hundreds of topological materials, and we need both intuition, experience, material-specific calculations, and intense experimental effort to finally find the right material for deeper exploration. And that led us on a decade-long journey to investigate many bismuth-based materials."
Insulators, like semiconductors, have so-called insulation, or band gaps: these are essentially "barriers" between orbital electrons, a kind of "no man's land" where electrons cannot enter. These band gaps are extremely important because, among other things, they are the key to overcoming the limitations imposed by thermal noise on the realization of quantum states.
They can do so if the width of the band gap exceeds the width of the thermal noise. However, an excessively large band gap has the potential to disrupt the spin-orbit coupling of the electron: this is the interaction between the electron's spin and its orbital motion around the nucleus. When this disruption occurs, the topological quantum state collapses. The trick to inducing and maintaining quantum effects is therefore to find a balance between large band gaps and spin-orbit coupling effects.
Hasan's team studied the bismuth bromide family of materials, and the team found that the properties of the bismuth bromide insulator made it more desirable than the bismuth-antimony-based topological insulators they had previously studied (Bi-Sb alloys): it had a large insulating gap of more than 200 meV (milli-electron volts). This is large enough to overcome thermal noise, but small enough not to destroy spin-orbit coupling effects and band-inversion topology.
Edge states at room temperature. a) Differential spectra taken from the surface and edge with temperature, represented by blue and red curves, respectively (positions marked in the corresponding topographic map images in b; blue curves are averages of the blue marked regions). b) Topographic maps and corresponding differential conductance maps taken at T=300, 200, and 100 K (V=0 mV) on the edge of a single ladder, capturing the temperature robustness of the edge states. c) Intensity maps of a series of line spectra taken along the a-axis direction at T=300 K (marked with red lines on the corresponding topography maps in b; scan directions are marked with arrows), showing the presence of gapless edge states at room temperature.
"In this case, we found a balance between spin-orbit coupling effects and large bandgap widths in our experiments." Hasan said, "We found that there is a 'sweet spot' where scientists can have relatively large spin-orbit coupling to create a topological distortion, as well as raise the band gap without breaking it. It's kind of like an equilibrium point for the bismuth-based materials we've been studying for a long time."
The researchers knew they had achieved their goal when they looked at the experiment through a subatomic-resolution scanning tunneling microscope, a unique device that uses a property known as "quantum tunneling," in which electrons are funneled between the microscope's sharp single-atom tip of the metal and the sample. The microscope uses this tunneling current rather than light to observe the world of electrons on the atomic scale. Ultimately, the researchers observed a clear quantum spin Hall edge state, which is one of the most important properties unique to topological systems.
Hasan said, "We have demonstrated for the first time that there is a class of bismuth-based topological materials whose topological structure persists at room temperature."
03The starting point for the future of nanotechnology: will accelerate the development of more efficient and green quantum materials
The discovery is the culmination of years of hard-won experimental work, and for 15 years Hasan has been a leading researcher in the field of experimental quantum topological materials with novel experimental methods; in fact, he was one of the early pioneering researchers in the field.
For example, between 2005 and 2007, he and his research team used novel experimental methods to discover topological order in three-dimensional bismuth-antimony bulk solids, semiconducting alloys, and related topological Dirac materials: this led to the discovery of topological magnetic materials; between 2014 and 2015, they discovered a new class of topological materials called magnetic exo-elastic semimetals.
The researchers believe that this breakthrough will open the door to a plethora of future research possibilities and applications of quantum technologies.
We believe this discovery could be the starting point for future developments in nanotechnology," said Shafayat Hossain, a postdoctoral research associate in Hasan's lab and another co-first author of the study. There are already many proposed possibilities in topological technology waiting in the wings, and finding the right materials coupled with novel instrumentation is the key to them."
Hasan and his team believe this breakthrough will have a particular impact on one area of research for the next generation of quantum technology: the accelerated development of more efficient, "green" quantum materials.
Currently, Hasan says, the team's theoretical and experimental focus is in two directions.
First, the researchers hope to determine what other topological materials might work at room temperature and, more importantly, to provide other scientists with tools and new instrumentation to identify materials that will work at room and high temperatures.
Second, the researchers hope to continue to probe deeper into the quantum world, as this discovery makes it possible to conduct experiments at higher temperatures.
"Our research is a real step forward in demonstrating the potential of topological materials for energy-efficient applications," Hasan added, "and what we are doing here with this experiment is planting a seed that will encourage other scientists and engineers to dream big."
Referencelinks:
[1]https://phys.org/news/2022-10-scientists-exotic-quantum-state-room.html
[2]https://www.nature.com/articles/s41563-022-01304-3
