Unprecedented: a quantum effect observed at room temperature

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

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 very conductive on the surface, which makes them sensitive to different quantum effects. However, thermal agitation makes the material entirely conductive in the volume, causing the diffusion of electrons between surface and volume, thus 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 succeeded in presenting its work in NatureMaterials.

It is in bismuth bromide (Bi4Br4), a crystalline inorganic compound, that the Princeton scientists found a sufficiently large “gap” (200 millielectronvolts) to allow a quantum effect to be maintained at room temperature.

In a material, the electrons can take on different energies that are well defined and 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 one (conduction band): if they are close enough, electrons can come and go between the two , and the material will be conductive. Conversely, a large gap will be the prerogative of an insulating material. It is therefore an essential property of the material, which governs the separation between the electrons: to avoid problems due to thermal agitation, a wide gap is necessary and makes it possible to separate the surface electrons from those inside. the topological insulator. But in addition, too large a gap would also disturb the quantum state. It is therefore a balancing act to find the right material, and this observation is a first in the field of topological insulators, recent materials but which are the subject of very active research, in particular for their quantum properties. Getting rid of a cooling 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 discussion about their potential for practical applications” says Zahid Hasan, a professor at Princeton University who led the study, “but until macroscopic quantum topological effects can be realized at room temperature, these applications will remain hypothetical”. This discovery is therefore an important step towards future applications.

“There is reason to rejoice: in many materials, a limiting factor is the existence of conduction pockets (electrons or holes) called “puddles” in the interior of the material. These effects are closely related to the size of the gap, and the larger the gap, the less the material will be sensitive to it” 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, the researcher tempers: “Even if this study is encouraging, we must be careful about the relevance of this material for the applications, because the study here is done under very specific conditions, in particular under ultra-high vacuum. The observation of a large gap is a first step that was taken by several materials, which nevertheless proved too difficult to use subsequently for the production of electronic devices.The material would have to resist oxidation, impurities and other defects would not do 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’ve done with this experiment is plant a seed to encourage other scientists to think big!”

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