Quantum Critical

CeCu6, by itself, is an ordinary nonmagnetic metal. But if you replace some of the copper atoms with gold atoms, its properties change. By adjusting the concentration of gold and the temperature, researchers can tune the material from a normal metallic state to a magnetic state as shown on this phase diagram. The boundaries between the two phases meet at a critical point at absolute zero. At the higher temperatures accessible to experiments, there is a region between the two phase boundaries in which CeCu6Au is neither a metal nor a magnet, just as opalescent water is neither a liquid nor a gas. In this region, CeCu6Au is "quantum critical," and it is fluctuating between a magnetic phase and a metallic one. This is the kind of emergent behavior Coleman wants to understand.