In addition, Brazil sees in twistronics – summaries

Graph is pop. Properly or improperly touted, the material’s technological potential is immense due to properties such as lightness, flexibility, hardness and ability to conduct electricity.

Despite all the turbulence, graphene is nothing more than an extremely thin layer of graphite that is only one atom thick. First isolated and characterized in detail in 2004, its properties have sparked a wave of studies worldwide, including other two-dimensional and lamellar materials.

“Materials with a lamellar structure such as graphene are diverse in nature. The soapstone from Aleijadinho’s sculptures, for example, has a lamellar structure. They appear three-dimensional to us because they are stacked like graphs in graphite. When we write with a pencil, we remove the graphite and mark our paper with graphs, ”explains Ado Jório from the Federal University of Minas Gerais (UFMG).

Jório heads the research group whose work with graphs took place on February 17th in one of the most controversial places in world science: the cover of the journal Nature. The property examined by the group was superconductivity, which occurs when two layers of graphene are stacked and one of them is rotated at an angle of exactly 1.1 degrees.

To understand the work of the Brazilian group we have a simple experience. Place your hands on top of each other, palm to palm. Now turn your right hand slightly and slide it over your left hand.

Nothing happens to our hands except the visual displacement of the fingers of one hand compared to the other. But when scientists do the same thing with two layers of graphene, a world of new possibilities emerges.

Graphene is a flat layer in which the carbon atoms are arranged in a network in a hexagonal structure. It is this crystalline structure – and consequently the electronic and vibrational structures – that give the material its unique properties.

“The electronic structure and the vibration structure together define almost all the properties of the materials,” says Jório. “Why does light pass through the lens in glasses but not through the stem? Why is the shirt you wear malleable but the frame of the glasses is rigid? Why is your mobile phone screen sensitive to touch? “Asks the researcher. “The answer to all of these questions lies in the electronic and vibrational structure of each material.”

Representation of the build-up by rotating one graphene layer on top of another (Photo credit: Ponor, CC BY-SA 4.0, via Wikimedia Commons)

The rotation (twist) of the graphene bilayer causes the network to become a super network in which the smaller hexagons of the original network become a larger hexagonal structure (as in the picture). The phenomenon of superconductivity resulting from this change was verified experimentally in 2018 and illustrates the emergence and potential of a new scientific and technological field, twistronics.

Ado Jório says that the term Twistronic is typical for two-dimensional materials. He explains how in a three-dimensional material – for example a cube – the properties of the structure are within this material. So if we connect two cubes and rotate them relative to each other, we can change something about the surfaces that are in contact, but not what is further inside. “However, if we take a material with an atom of thickness and touch another, the influence is very great, and the orientation with which we include the second graphene layer, the angle, for example, plays a fundamental role,” he explains.

In the case of graphene, it is the rotation of exactly 1.1 degrees that makes the material superconducting. Although this was verified in 2018, there is still no theoretical model to understand why the phenomenon occurs. This is important in order to be able to control it and one day be able to apply it technologically in devices of daily use. In order to understand what is happening, the contribution of the article published in Nature comes in this direction, the results of which were only possible thanks to equipment developed here in Brazil: the nanoscope.

Jório says that what explains superconductivity – that is, the existence of materials that conduct electricity without resistance, and therefore without losses – is the way in which the electronic particle moving through the material interacts with the species and how the material vibrates. “For the first time, the nanoscope made it possible to generate images and characterizations of the electronic structure and the vibrational structure with a resolution exactly on the nanoscopic scale. Now other researchers have the data to develop a theoretical model to explain superconductivity in the round graphene bilayer, based on the electronic and vibrational properties that we show for what they are, ”explains the researcher.

The resolution of microscopes doesn’t allow you to see anything smaller than a micron. So the gain of the nanoscope is precisely the ability to see structures and phenomena that occur on the order of nanometers, that is, on a scale a thousand times smaller than that of the micrometer.

The capacity of the nanoscope essentially depends on the size of the antenna with which the examined material is analyzed. “We developed a nano antenna with a specific technology that we developed. This nano antenna led to a much better function than any other existing nanoscope in the world and thus to images that are as informative and rich as those mentioned in the article, ”says the professor at UFMG.

“On the other hand, the challenge with mathematical modeling lies in the fact that the structures in the supernet are large and therefore require a lot of computing capacity,” adds Jório. “What our article brings very valuable is the gain in resolution from an experimental point of view as well as the fact that the theorists who worked with us have created a model that can be used to calculate very large structures for which no other has existed so far could do. “

Despite the importance of the results for the further development of twistronics, there will not be graphene bilayers that conduct energy tomorrow.

“From the emergence of a new proposal that is Twistronics, to the ability to dominate the production of this type of material in a way that is robust enough to be used in technological applications, there is still a lot of research to be done and a lot of engineering work ahead, ”explains Ado. Jório. “It is necessary to have the material rotate at that exact angle, at the desired size, within the desired device, and in a stable manner, without returning it to its original position. For you to be in our daily life, I estimate a range from 10 to 50 years. I don’t know if 10 or 50, but I doubt it will happen in 5 years, ”reveals the researcher. “But the nanoscope is a technological reality in the present!” He concludes.

In addition to the UFMG group, which is made up of researchers and students from various fields, employees of the Federal University of Bahia, Inmetro (National Institute for Metrology, Quality and Technology) and partner institutions in Japan, the USA and Belgium.

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