a sparsely filled √13x√13 superlattice and rs=70~100 may be considered to be better described in terms of a Wigner crystal than the more traditional charge density wave. This viewpoint is supported both by modelling and systematic scanning tunnelling microscopy measurements. Thus, Wigner crystal superlattices in so-called CDW systems may be considered to be the first direct observation of ordered electron states localised by mutual Coulomb interaction. An important criterion for is the depth of charge modulation, which depends on the material, and only systems where rs exceeds the theoretical limit can be regarded as Wigner crystals. In 2020 a direct image of a Wigner crystal observed by microscopy was obtained in molybdenum diselenide/molybdenum disulfide (WSe2/WS2) moiré heterostructures. A 2021 experiment created a Wigner crystal near 0K by confining electrons using a monolayer sheet of molybdenum diselenide. The sheet was sandwiched between two graphene electrodes and a voltage was applied. The resulting electron spacing was around 20 nanometers, as measured by the stationary appearance of light-agitated excitons. Another 2021 experiment reported quantum Wigner crystals where quantum fluctuations dominate over the thermal fluctuation in two coupled layers of molybdenum diselenide without any magnetic field. The researchers documented both thermal and quantum melting of the Wigner crystal in this experiment. References Condensed matter physics