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Mixed-order phase transition in a colloidal crystal

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Phase transitions are usually classified in two categories: First-order transitions are discontinuous, and second-order transitions are continuous and exhibit critical behavior. However, the so-called mixed-order transitions combine features of both types, such as being discontinuous yet featuring a diverging correlation length. Such transitions were first foreseen by David J. Thouless in 1969 in spin chains with long-range interactions. Since then, they have been predicted for a number of equilibrium and nonequilibrium systems. Here, we predict and experimentally observe a mixed-order phase transition in a colloidal crystal. Our findings constitute an experimental realization of an equilibrium mixed-order transition, thus enabling the experimental investigation of the surprising properties of such phase transitions.