Science

New quantum state observed at room temperature could revolutionize electronics

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The search for new topological properties of matter is a new gold rush in modern physics. For the first time, physicists have observed new quantum effects in a topological insulator based on the element bismuth at room temperature. This discovery opens up new possibilities for the development of efficient and energy-efficient quantum technologies.

In recent years, the study of the topological states of matter has attracted considerable attention from physicists and engineers and is currently a subject of great international interest and research. This area of ​​research combines quantum physics with topology, a branch of theoretical mathematics that studies geometric properties that can be deformed but not fundamentally changed.

In other words, topology is the branch of mathematics that studies the properties of geometric objects that remain under continuous deformation without tearing or sticking together, like a rubber band that can be stretched without breaking.

Dr. Zahid Hasan, professor of physics at Princeton University, lead author of this study, emphasizes in a press release: “The new topological properties of matter have become one of the most sought-after treasures of modern physics, both from a fundamental physics point of view and find potential applications in quantum engineering and nanotechnology. next generation.”

As a result, spintronics arose. It is based on using a fundamental property of particles known as spin to process information. Spin is a quantum property of particles that is closely related to their rotational properties. It plays an essential role in the properties of matter.

Spintronics is similar to electronics, the latter uses the electric charge of an electron instead of spin. The transmission of information about both the charge and rotation of an electron potentially offers devices with a wide variety of functions.

Princeton researchers have found that a topological insulator-like material made from the elements bismuth and bromine exhibits quantum behavior observed only under extreme experimental conditions of high pressure and temperatures close to absolute zero. This discovery opens up new possibilities for the development of efficient quantum technologies based on spintronics. Their work is published in the journal Nature Materials.

World’s first at room temperature

It should be noted that scientists have been using topological insulators to demonstrate quantum effects for more than a decade. It is a unique device that acts as an insulator within the volume—the electrons inside the insulator cannot move freely and therefore do not conduct electricity—but whose surface can nevertheless become a driver.

The experiment described in this study is the first in which they were observed at room temperature. Usually, temperatures close to absolute zero (about -273 degrees Celsius) are required to induce and observe quantum states in topological insulators.

Indeed, the environment or high temperatures create what physicists call “thermal noise,” defined as an increase in temperature at which atoms begin to vibrate violently. This action can disrupt the operation of subtle quantum systems, thereby destroying the quantum state.

Particularly in topological insulators, these higher temperatures create a situation where electrons on the surface of the insulator invade the bulk of the insulator and cause the electrons to also begin to conduct, which weakens or breaks the special quantum effect.

Therefore, the solution is to subject these experiments to extremely low temperatures, usually at or near absolute zero. At these temperatures, atomic and subatomic particles stop vibrating and are therefore easier to manipulate. However, the creation and maintenance of an ultracold environment is impractical for many reasons: cost, volume, high energy consumption.

Unique topological insulator

Hasan and his team have come up with an innovative way to solve this problem. Drawing on their experience with topological materials, they produced a new type of topological insulator based on bismuth bromide, an inorganic crystalline compound sometimes used in water purification and chemical analysis.

Specifically, you should know that insulators, like semiconductors, have so-called insulating (or forbidden bands). In fact, these are “barriers” between rotating electrons, a kind of “neutral strip” where electrons cannot get, the authors explain. These band gaps are extremely important as they provide the cornerstone for overcoming the quantum state limitation imposed by thermal noise.

However, they occur if the band gap exceeds the thermal noise width. But a band gap that is too large can potentially disrupt the electron’s spin-orbit coupling, which is the interaction between the electron’s spin and its orbital motion around the nucleus. When this perturbation occurs, the topological quantum state collapses. So the trick to creating and maintaining a quantum effect is to find a balance between the wide bandgap and spin-orbit coupling effects.

The insulator that Hasan and his team studied has an insulating gap of more than 200 meV, large enough to overcome thermal noise, but small enough not to disturb the spin-orbit coupling effect and the inversion topology of the bands.

Revolutionary discovery for electronics

Hassan says: “In our experiments, we have found a balance between spin-orbit coupling effects and a large band gap. We found that there is a “best place” where there can be a relatively large spin-orbit coupling in order to create a topological turn and increase the band gap without destroying it. It’s sort of a balance point for bismuth-based materials, which we’ve been studying for a long time.”

To highlight this property, the researchers used a subatomic resolution scanning tunneling microscope, a unique device that exploits a property known as “quantum tunneling.” In particular, when a single-atom microscope tip approaches the surface at a distance of 1 nm, the needle’s electrons do not want to remain on the tip and can be transferred to the surface, illustrating the tunneling effect. The microscope determines the electrical conductivity between the tip and the surface, that is, the amount of current passing through it. Scanning line by line, we obtain an electronic map of the surface and each atom or molecule placed on it.

This is how the researchers observed a distinct quantum Hall edge spin state, which is one of the important properties that only exist in topological systems. This required additional new tools to single out the topological effect unambiguously.

Nana Shumiya, a postdoctoral fellow in electrical and computer engineering and one of the study’s three co-authors, explains: “It’s just great that we found them without the giant pressure or ultra-high magnetic field, thereby making the materials more accessible. to develop next-generation quantum technologies.” She adds: “I believe that our discovery will significantly advance the quantum frontier.”

The researchers now want to determine which other topological materials can work at room temperature and, most importantly, provide other scientists with the tools and new measurement methods to identify materials that can work at room temperature and elevated temperatures.

Materials of nature

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