Unprecedented: a quantum effect observed at room temperature

Quantum physics is full of original and unusual effects, which leave you wondering about its possible applications. And if some technologies, such as sensors or quantum clocks, are already having great success, others, such as the famous quantum computer, come up against very strong technical limitations. Among the latter, the current need to place the materials that are the seat of quantum effects at extremely low temperatures, close to absolute zero (-273.15°C). The particular failure of thermal agitation: as soon as the temperature is increased, the atoms begin to vibrate, the electrons to move, and this movement breaks the fragile quantum state in which a material is found.

Thermal agitation undermines quantum effects

For example, topological insulators are materials that are bulk insulators (electrons inside are not free to move, so electric current does not flow) but highly conductive on the surface, making them sensitive to different quantum effects. . However, thermal agitation makes the material completely conductive in volume, causing diffusion of electrons between the surface and the volume, causing it to lose its quantum properties. The challenge is, therefore, to find a material that resists these conditions, and this is what the team has achieved by presenting their work in Nature Materials.

It is in bismuth bromide (Bi4brother4), a crystalline inorganic compound, that the Princeton scientists found a “gap” large enough (200 millielectron volts) to allow a quantum effect to be maintained at room temperature.

In a material, electrons can take on different well-defined energies distributed in levels called “bands”. The gap is the amount of energy that separates the last band completely filled with electrons (valence band) and the next (conduction band): if they are close enough, electrons can go back and forth between the two, and the material will be driver . On the contrary, a large space will be the prerogative of an insulating material. Therefore, it is an essential property of the material that governs the separation between the electrons: to avoid problems due to thermal agitation, a wide space is necessary and allows the separation of the electrons on the surface from those inside the topological insulator. But in addition, too large a gap would also perturb the quantum state. Finding the right material is therefore a balancing act, and this observation is a first in the field of topological insulators, materials that are recent but are the subject of very active research, particularly for their quantum properties. Getting rid of a refrigeration system that is costly in terms of space and energy would therefore be a considerable advance in the field.

A world of materials to explore

“Topological materials are attracting a lot of interest and debate about their potential for practical applications” says Zahid Hasan, a Princeton University professor who led the study, “But until macroscopic quantum topological effects can be realized at room temperature, these applications will remain hypothetical.”. Therefore, this discovery is an important step towards future applications.

“There is reason to rejoice: in many materials, a limiting factor is the existence of conducting pockets (electrons or holes) called “puddles” inside the material. These effects are closely related to the size of the gap, and the quantum The larger the space, the less sensitive the material.” explain to Science and Future Erwann Bocquillon, a professor at the University of Cologne who was not involved in the study.

But there is still a long way to go before we see a revolution in the use of quantum materials, tempers the researcher: “Even if this study is encouraging, we need to be careful about the relevance of this material to applications, because the study here is done under very specific conditions, in particular under ultrahigh vacuum. The observation of a large gap is a first step that they took.” various materials, which however proved too difficult to later use for the production of electronic devices.The material would have to resist oxidation, impurities and other defects would not make them conductive… In short, a large gap does not predict a perfect material in electronic transport. But the team behind the study is not going to stop there, and intends to explore new materials with similar properties and develop new techniques to characterize them. Thus, for Zahid Hasan, “What we have done with this experiment is plant a seed to encourage other scientists to think big!”

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