SCIENTIFIC EDUCATIONAL CENTER science idea

For the first time in the history of science, a group of researchers from the Swiss Federal Institute of Technology in Zurich (ETH Zurich) managed to form an exotic crystal consisting exclusively of electrons on the surface of a semiconductor material and observe it. With this achievement, they confirmed the theoretical forecast put forward about 90 years ago, in 1934, by Eugene Wigner, which throughout this time has been one of the main challenges in the field of condensed matter physics.

In 1934, Eugene Wigner, the founder of the theory of symmetries in quantum mechanics, demonstrated that electrons in the medium of a material with certain electrical properties can organize themselves and create an image resembling a crystal lattice. This should have happened when the potential energy of the mutual repulsion of the electrons is greater than the kinetic energy of their movement. And the electrons will arrange themselves in such a way that their total energy will be as small as possible. Such "Wigner crystals" remained only a theoretical concept for a long time, because they can form only at extremely low temperatures, the number of free electrons is very small in any material and the electrons themselves are very small. This, in turn, means that the kinetic energy of the electrons in the normal state will always be much greater than the energy of the electrostatic interactions between them.

To create the "Wigner crystal", scientists used the thinnest layer of a semiconductor material — molybdenum diselenide. Since this layer has a thickness of one atom, the electrons in the material can only move within one two-dimensional plane. The researchers increased the number of free electrons by applying an electric potential to the material through graphene electrodes pressed against it. And, according to theoretical calculations, the unique electronic properties of molybdenum diselenide at a temperature close to absolute zero should contribute to the appearance of"Wigner crystals".

However, the increased probability of the occurrence of a" Wigner crystal " is not quite enough to make a discovery. Scientists needed to prove that such crystals actually appeared and to investigate some of their properties. This is complicated by the fact that the distance between the electrons in the crystal lattice is about 20 nanometers, thirty times less than the wavelength of light in the visible range, which makes it impossible to directly observe this phenomenon using even the most advanced microscopes.

In order to" visualize "the electrons of the Wigner crystal, scientists used laser light with a certain frequency, which excited so-called excitons, quasiparticles consisting of an interconnected free electron and an electron hole, on the surface of the material. Knowing the frequency of excitons, the direction and speed of their movement, the scientists determined the effects of the interaction of excitons with other free electrons in the material.

As a result of the interaction of excitons and electrons, an effect occurs that is very similar to the effect that occurs when a rotating wheel of a car, for example, gets into the lens of a video or movie camera. When the car reaches a certain speed, an optical illusion occurs, because of which it first seems to the viewer that the wheel of the car first stops completely, and then begins to rotate in the opposite direction. This happens when the visible element of the wheel disk is moved to the place of another such element in the time for which the camera takes one frame, 40 ms. The same thing happens with excitons moving at a certain speed, they first "freeze in place", and then it seems that they begin to move in the opposite direction.

And in conclusion, it should be noted that this case is the first time in history when scientists have obtained direct proof of the regular arrangement of electrons in a semiconductor material. Yes, there were other similar attempts earlier, but in all of them scientists used only indirect measurement methods, the reliability of the results of which is under a big question mark.

In their further research, Swiss scientists plan to use the technology developed by them to observe the process of formation of" Wigner crystals "from an" electronic liquid " that has a completely disorderly nature.

The article was published in the journal Nature

Photo: Wigner crystal made of electrons (red) inside a semiconductor material (blue / gray) © ETH Zurich
Source: dailytechinfo.org, sci-dig.ru