= Electron crystallization on liquid helium
The 1979 experiment of <C. C. Grimes> and colleagues studied a two-dimensional sheet of electrons above liquid helium. It reported crystallization at densities $3\times10^8$ to $9\times10^8\,\mathrm{cm}^{-2}$, with melting temperatures $0.35$ to $0.65\,\mathrm K$. The sheet was classical: Coulomb energy competed with thermal motion, unlike the kinetic-energy comparison used for a zero-temperature degenerate <Wigner crystal>. It supplies evidence for electron ordering while keeping these two physical regimes distinct.<Evidence for a Liquid-to-Crystal Phase Transition in a Classical, Two-Dimensional Sheet of Electrons by C. C. Grimes and others>[Physical Review Letters 42, pp. 795–798, abstract: classical sheet and reported density and melting-temperature intervals]{ref}
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