Scientists Observe Quantum Spin Hall Edge State At Room Temperature

Scientists Observe Quantum Spin Hall Edge State At Room Temperature

Princeton University researchers have discovered that a material known as a topological insulator, composed of bismuth, and bromine, exhibits unique quantum behaviours typically observed only under extreme experimental conditions such as high pressures and near absolute zero temperature. This discovery lays the foundation for the future development of efficient quantum technologies like spin-based, high-energy-efficiency circuits.

Topological materials are physical states of matter with unique properties that do not change even when deformed. An example is topological insulators, the primary device component scientists use to explore quantum topology. 

The inside of the topological insulator acts as an insulator, preventing the electrons from freely moving about and conducting electricity. However, the electrons on the device’s edges move freely, indicating that they are conductive. The electrons on the edges cannot be deformed. This device can improve technology and provide a better understanding of matter by exploring quantum electronic characteristics.

Since the quantum Hall effect of 1985, topological phases have been the subject of extensive research. Topological insulators, Weyl semimetals, and several other classes of quantum materials with topological electronic structures have been discovered. F. Duncan Haldane, the Sherman Fairchild University Professor of Physics at Princeton, advanced significant theoretical ideas on two-dimensional (2D) topological insulators in 1988.

Princeton University researchers led by M. Zahid Hasan, the Eugene Higgins Professor of Physics at Princeton University, have been using topological insulators to demonstrate quantum effects for over a decade, but this is the first time this behavior has been observed at room temperature. In most cases, generating and monitoring quantum states in topological insulators requires temperatures close to absolute zero or minus 459 degrees Fahrenheit.

Even with years of research, topological materials aren’t applied in devices. This is due to a temperature spike that causes the atoms to vibrate wildly; physicists term this thermal noise. Thermal noise could disturb fragile quantum systems, and the quantum state collapses.

In topological insulators, high temperatures cause the electrons on the insulator’s surface to invade the bulk of the insulator, causing them to begin conducting, thereby breaking the quantum effect. 

The solution is to conduct the experiments at extremely low temperatures or near absolute zero. Atomic and subatomic particles stop vibrating at these extremely low temperatures, making them easier to handle. But creating and sustaining an ultra-cold environment is impracticable for many purposes; it is energy-consuming and expensive. These researchers from Princeton have devised a new solution to this challenge. 

In response to a suggestion by collaborators and co-authors Fan Zhang and Yugui Yao to study a particular class of Weyl metals, Hasan and his colleagues began experimenting with the family of compounds called bismuth bromide. 

The Weyl phenomenon, however, was not visible. Instead, Hasan and his colleagues found that the Bismuth Bromide insulator has more desirable characteristics than the topological insulators (Bi-Sb alloys) they had previously studied.

Bismuth Bromide has a large insulating gap of over 200 meV (“milli electron volts”), big enough to cancel out thermal noise and small enough to not interfere with the band inversion topology or the spin-orbit coupling effect.

The researchers viewed the material through a sub-atomic resolution scanning tunneling microscope. The device uses a property known as “quantum tunneling,” where electrons are funneled between the sharp metallic tip of the microscope and the sample. The microscope uses this tunneling current rather than light to view electrons on the atomic scale.

They observed a clear quantum spin Hall edge state, one of the important properties that exist in topological systems. Their paper, Evidence of a room-temperature quantum spin Hall edge state in a higher-order topological insulator, was published in Nature Materials on July 14, 2022. 

“For the first time, we demonstrated that there’s a class of bismuth-based topological materials that the topology survives up to room temperature,” “We are very confident of our result.”

Zahid Hasan

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