Watching electronic ice melt.

One of the most familiar phase transitions on Earth is the freezing and melting of water. Underlying this transition is the competition between the kinetic energy of water molecules and the potential energy arising from their mutual interactions. When the temperature falls below the freezing point, the interaction energy wins this competition and water freezes. A two-dimensional (2D) system of electrons exhibits a similar competition between kinetic and potential energies that also gives rise to a freezing–melting transition. However, the nature of this transition in an electron system has remained mysterious for decades. On page 736 of this issue, Xiang et al. (1) report direct imaging of the melting and freezing of a Wigner crystal—a periodic 2D lattice of strongly interacting electrons. The observed intricate spatial patterns raise fascinating questions about the nature of this fundamental phase transition in the quantum realm. [ABSTRACT FROM AUTHOR]