Evidence for a Liquid-to-Crystal Phase Transition in a Classical, Two-Dimensional Sheet of Electrons

Experimental evidence is presented for an electron-liquid to electron-crystal phase transition in a sheet of electrons on a liquid-He surface. The phase transition has been studied for electron areal densities from 3×{}10⁸ cm^-2 to 9×{}10⁸ cm^-2 and has yielded melting temperatures between 0.35 and 0.65 K. The phase transition occurs at Γ=137±15, where Γ is a measure of the ratio of potential energy to kinetic energy per electron.