Exotic structures known as "space–time crystals" could soon become reality, thanks to research carried out by a collaboration of physicists in the US and China. The researchers, developing an idea put forward by theoretical physicist Frank Wilczek, of the Massachusetts Institute of Technology in the US, have shown how a crystal made of trapped ions could rotate persistently, even when in its lowest energy state, enabling it to break temporal as well as spatial symmetry.
Normal crystals consist of atoms or molecules arranged in ordered 3D structures, which form below a certain temperature in order to minimize the potential energy within the material. The carbon atoms in diamond, for example, experience an attractive force at longer distances but a repulsive force at smaller scales, meaning that their energy is at a minimum when they are separated from one another by approximately the same intermediate distance. Contrastingly, at higher temperatures, the atoms in a crystal can exist in many more disordered states than ordered states and if the former tends to prevail, it causes the material to melt. "Crystals are a victory of energy over entropy," says Wilczek.
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