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A crystal made of electrons

Scientists have produced a very special crystal. Unlike normal crystals, it is made up exclusively of electrons. In doing so, they have corroborated a theoretical prediction that was made in 1934.

The experiment in which such a unique crystal has been generated has been carried out by a team that includes, among others, Tomasz Smolenski and Atac Imamoglu, both from the Institute of Quantum Electronics under the Swiss Federal Institute of Technology in Zurich (ETH).

In 1934, Eugene Wigner, one of the founders of the theory of symmetries in quantum mechanics, theoretically showed that the electrons in a material could be arranged in regular patterns, analogous to those of a conventional crystal, due to their electrical repulsion. mutual. The reasoning is very simple: if the energy of electrical repulsion between electrons is greater than their energy of motion, they will be arranged so that their total energy is as low as possible.

However, for several decades this prediction remained purely theoretical, as these “Wigner crystals” can only form under extreme conditions, such as very low temperatures and a very small number of free electrons in the material. This is partly because electrons are many thousands of times lighter than atoms, which means that their energy of motion in a regular arrangement is usually much greater than the electrostatic energy due to the interaction between electrons.

To overcome these obstacles, Imamoglu and his collaborators chose for this new demonstration a thin layer of the semiconductor material molybdenum diselenide. The layer is so thin that it is only one atom thick. Because of this, electrons can only move in one plane.

Graphic representation of a Wigner crystal (in red), made of electrons, in a semiconductor material (blue / gray) one atom thick. (Image: ETH Zurich)

The researchers managed to vary the amount of free electrons by applying a voltage to two transparent graphene electrodes, between which they had inserted the semiconductor.

The experimental setup was cooled down to just a few degrees Celsius above Absolute Zero. The latter is the lowest temperature allowed by the laws of physics and is about 273 degrees Celsius below zero.

The low temperature and electrical properties of molybdenum diselenide eventually caused the formation of a Wigner crystal.

The presence of this structure was verified by various techniques.

The researchers have published the details of their experiment in the academic journal Nature, under the title “Signatures of Wigner crystal of electrons in a monolayer semiconductor.” (Source: NCYT from Amazings)

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