The potential existence of so-called Wigner crystals was predicted almost ninety years ago but have only now been observed directly in a semiconductor material.
A team of researchers led by Ataç Imamoğlu, professor at the Institute for Quantum Electronics at ETH Zurich made the observation in the single atomic layer semiconductor material molybdenum diselenide.
This 2-dimensional material constrains electrons to move within a plane and usually they do this as a disordered liquid but the electrons can form a regular Wigner crystal with fixed distances and between the electrons and angular arrangements because of electrical repulsion, under certain conditions.
The team's results were recently published in the scientific journal Nature.
The necessary condition is that their motional energy needs to be smaller than the electrical repulsion. In that case the electrons will arrange themselves to minimize the total energy of the system. This has required extremely low temperatures and very few free electrons in a system.
Theory predicted distances between electrons in molybdenum diselenide of about 20nm when taking the material to within a few degress of absolute zero.
The ETH Zurich team used a specific light frequency to create excitons – electron-hole pairs – in the material and an interference effect to reveal the presence of the Wigner crystal.
Related links and articles:
Smoleński T, Dolgirev PE, Kuhlenkamp C et al. Signatures of Wigner crystal of electrons in a monolayer semiconductor . Nature 595 , 53–57 (2021) .DOI: 10.1038/s41586-021-03590-4