According to foreign media reports, crystals have been attracting people since ancient times. The magic doesn't stop even if one knows that it's all caused by the simple interaction of attraction and repulsion between atoms and electrons. Now, a research team led by Professor Ata? Imamo?lu from the Institute of Quantum Electronics at ETH Zurich has now produced a very special crystal.
Unlike ordinary crystals, it is composed entirely of electrons. In doing so, Imamo?lu and their colleagues confirmed a theoretical prediction made nearly 90 years ago. It is reported that this prediction has since been regarded as the Holy Grail of condensed matter physics. Their findings were recently published in Nature.
Prophecies from Decades Ago
"What excites us is the simplicity of the problem," says Imamo?lu. As early as 1934, Eugene Wigner, one of the founders of the symmetry theory of quantum mechanics, pointed out that due to the mutual electrical repulsion between electrons, theoretically speaking, electrons in materials can move in a regular, crystal-like manner. arranged in a way. The reason behind this is simple: if the electrical repulsion between electrons is greater than the energy of their motion, they will arrange themselves in such a way that their total energy is as small as possible.
For decades, however, this prediction remained purely theoretical, because those "Wigner crystals" could only form under extreme conditions, such as low temperatures and a very small number of free electrons in the material. This is partly because electrons are thousands of times lighter than atoms, which means that the energy of their movement in regular arrangements is often much greater than the electrostatic energy generated by the interactions between electrons.
Electrons in the Plane
To overcome these obstacles, Imamo?lu and his collaborators chose a wafer-thin layer of the semiconductor material molybdenum diselenide, just one atom thick , therefore, electrons can only move in one plane. Researchers can change the number of free electrons by applying a voltage across two transparent graphene electrodes, which sandwich a semiconductor between them. According to theoretical considerations, the electrical properties of molybdenum diselenide should favor the formation of Wigner crystals - when the entire device is cooled to a few degrees above absolute zero - 273.15 degrees Celsius.
However, simply producing Wigner crystals is not enough. "The next problem is to prove that we actually have a Wigner crystal in our instrument," says Tomasz Smoleński. He is the first author of this study and a postdoctoral fellow in Imamo?lu's laboratory. It has been calculated that the distance between electrons is about 20 nanometers, about 30 times the wavelength of visible light, making it impossible to resolve even with the best microscopes.
By exciton detection
Physicists managed to make the regular arrangement of electrons visible despite tiny separations in the crystal lattice. To do this, they use light of a specific frequency to excite so-called excitons in the semiconductor layer. An exciton is a pair of electrons and a "hole" - the latter created by a missing electron in one of the material's energy levels. The precise frequency of light that generates excitons and the speed at which they move depend both on the properties of the material and on interactions with other electrons in the material, such as with Wigner crystals.
The periodic arrangement of electrons in a crystal creates an effect that can sometimes be seen on television. When a bicycle or car goes faster and faster than a certain speed, the wheels appear to be stationary and then turn in the opposite direction. This is because the camera takes a snapshot of the wheel every 40 milliseconds. If during this time a wheel with regularly spaced spokes has moved the precise distance between the spokes, the wheel will appear to no longer rotate. Likewise, in the presence of a Wigner crystal, moving excitons appear to be stationary as long as they move at a certain speed determined by the separation of electrons in the crystal lattice.
First direct observation
"A team of theoretical physicists, led by Eugene Demler of Harvard University who will travel to ETH this year, has theoretically calculated how the effect would This shows up in the observed exciton excitation frequencies - exactly what we observed in the laboratory," said Imamo?lu.
Compared with previous experiments based on planar semiconductors, the indirect observation of Wigner crystals through current measurements is a direct confirmation of the regular arrangement of electrons in the crystal. In the future, with this new method, Imamo?lu and his colleagues hope to study exactly how Wigner crystals form from disordered electron "liquids."